I think there's two ways to view achievements (as Paradox does them, at least).
One view is that the difficulty of an achievement is an inherent part of the achievement (hence the name "achievement"), and circumventing that difficulty by mods or by console cheats violates the validity of the achievement. This view would say that the difficult achievements should be gotten only if you have the skill, and the zany achievements gotten only if you actually encounter a zany scenario.
The other is that achievements are nothing more than fun targets for gameplay, and that it doesn't matter how you get there. This view would say that it doesn't matter if you mod or cheat your way to the achievement because achievements are just a fun bonus for each individual player, isolated to that specific player.
I would prefer they go the second way, but the important bit is that Paradox apparently prefers the first style, and therefore locks achievements behind "validity walls."
And as long as they're going with that approach, they can't allow mods that change province layout. That would allow you to, for example, get an achievement that said "own states X, Y, and Z" by modding your game so that those states contained various random provinces that were easy to get or that you already had.