At the risk of not being current, but I think the question deserves some answering.
They are mostly fighting about what they consider "plausible" instead of "fantastic" alternate history (AH). Problem for me is, that it is very much dependent on the biases and morality of the one making the statement. Which probably also confuses you.
For example, imagine telling a frenchman in 1920 that in a hundred years their closest ally would be Germany, who'd they'll fight another war with and have their greatest war hero lay down a common wreath at Verdun with the german chancellor shortly after. And the rest of the world would be talk of the "Axis Paris-Berlin". If they don't resort to violence, you'd at least be called crazy and a phantast and be reassured that the germans have always have been and forever will be the enemy of the french. And that it is more likely that the french border will be on the Rhine than your crazy scenario. More arguments that are likely: After all, there are plenty rhenish separatists and there is historical precedent. And that thing called the German Reich is just a measly 50 years old. An artificial construct as the natural state of Germany is a lot of little states with some under the benevolent protection of France.
A bit more abstract/structured with a few term definitions first:
- OTL (Original Time Line): History as it has happened in "real" history. Notably, this is the exact sequence of events. Which means a WW2 which does not involve a battle of Stalingrad (instead a confrontation happens in Saratov) does NOT qualify. If you want, you can refer a world wide war which involves USA/UK/FR/UDssR vs GER/ITA/JAP as OTL-ish, imho you should not expect it to behave anything like history. Which leads us to the
- ATL (Alternate Time Line): Which is any sequence of events not OTL (which means, by definition, every game of HoI4 is an ATL - in turn rendering complaints like "There is no guaranteed battle of Stalingrad" moot, because they are not even wrong). If an event has a "rough pendant" in OTL, you can prefix it with "alt-" (Which, when applied to the statement of @TalyonUngol renders his statement both true [since Germany didn't win OTL WW2] and false [since there is goodly amount of data which implies that Germany could win an alt-WW2 and would not even need an undue amount of luck for it]). A classical fallacy in this range is that people see outcomes of OTL and do not question why that outcome came about, instead setting it as an immutable characteristic of that actor. An ATL diverges at the:
- PoD (Point of Divergence): The point(s) in time an ATL starts diverging from OTL. This can be a single event (Princips weapon jams a second time leading Franz Ferdiand to survive, averting the WWs and reforming AH into a Danubian Federation), broad strokes which can't be pinned down to a single item (over time the various skirmishes in Weimar go more in favor of the communists, leading to a Red Reich which triggers a Red Tide in alliance with the UdSSR) or a set of distinct but indepenent events (which are not broad enough to cause significant diversion on their own, but add up. Sorry, no example)
In general, all PoDs are subject to the want-of-a-nail rule. This means, the further you go from a PoD the more doors open. Which means if I deal with a PoD of 1900, 2020 can look unrecognizably different. Which often a subject of contention, how fast and how far those doors open.
For me there is also what I call the hollywood collorary, which states that the more an ATL diverges from the narratives of OTL more hostility it will recieve. For example , many ATLs feature the USA going onto a sudden anti-fascist crusade in 1942 (or 1945), despite having diverged significantly before (say, President Lindbergh). If I had to guess, this is due to the authors wanting the USA to be on the "right side of history" as the were OTL, while forgetting that history is written by the winners. Which leads scenarios such as an alt-1940s where everything but a constitutional monarchy is seen as irrational in in Europe, and a presidential republic is "an american thing" seldomly being considered. We even have a thread in these forums about that disconnect currently.
Personally, I think few people are able to rationally handle a PoD previous to 1945 and will subconsciously try to nudge it toward an outcome similar to OTL. When you go pre-1914 you'll get a lot of flack for making scenarios where the Germans are not moustache-twirling villains or immediately comply with anything the UK/France/Russia demand of them.
- One more thing: The concept of escalating advantage (also known as Matthew effect) which OTL is traditionally the domain of the USA. If somebody else (say, the Germans in an alt-Barbarossa) suddenly profits from it, people often have problems reconciling it with their expectation of "proper" alternate history. In my eyes, this is a continuation of the fallacy that ascribes outcomes as inherent values to actors without acknowledging how those outcomes came about (namely in OTL, the germans f-ing up and having to divide their forces while the Soviets rallied while being aided by the WAllies). Which makes statements like "any alt-WW2" indefensible.
And this is before nationalism of the author or statistics "modified" by a government come into play.
So, having written all that:
As previously stated, the statement "There is this ridiculous theory that Germany could have won ww2." is both true and false (since Germany did loose OTL WW2, while the assertion that Germany would loose any alt-WW2 is a bit of a stretch).
The other bit is: " totally fantastic history, for example recreating a new Byzantine empire in which the Greeks or Bulgarians of the mid-30s never or if raised in any way" which is correct (it wasn't raised in OTL) but misleading (since if a fascist/revanchist Greece does well enough with a PoD around 1936, you bet the will seize upon that mantle). They just never did well enough in OTL to entertain that thought.
For HOI4, that means the more "out there" they should be low on the focus tree or locked behind stiff prerequesites. And there is the problem that HOI4 has no prologue to model diversions prior to 1936. Or just implicitly via the first focus in a tree.
As for your original assertion: Many hardcore WW2-games outright deny the escalating advantage, often by offering only a set of scenarios or outright wiping any advantage you might have gained. This is probably an attempt to enforce the outcome "Germany looses" no matter how good the German player is or how much the soviet player fs-up. Which makes these kind of games uninteresting to me, since I have no love for counting bullets and blankes or reenacting OTL.
Edit: Good god, that got long oO