They also said that Italy being a major was a bug....at least Daniel said that.
That was a joke!
Guys, this is simple. There are seven countries which had highly developed industry, technical research and access to resources, and had great international influence. China wasn't one of them in 1936, but has the potential to become a major power. The first thing blocking China is factionalism and civil war, that had been plaguing the country since 1911.
The country was also impoverished by the series of wars with the major powers starting in 1839 with the First Opium War - every defeat (and there were many) cost it huge amounts of silver in reparations. And suffered from the Unequal Treaties imposed on it as a result of those wars where most foreign trade was being carried out by the major powers from the Treaty Ports through the international settlements. Plus they had lost ownership of or influence over Vladivostok and Outer Manchuria, Manchuria, Korea, Mongolia, North Vietnam, Dalian, Hong Kong, Weiheiwei, Macau and Guangzhouwan, during what became known as the "Century of Humiliation".
China had the capability to field very large armies, but was inward looking, and had no navy. It was primarily the reason it suffered so many defeats to the major powers - it was incapable of preventing them from sailing into their ports and bombarding them, landing amphibious invasions, or sinking ships carrying Chinese trade goods. In fact, you could almost define a major power as one with a substantial navy. Not only because of the ability to defend themselves and project their power overseas, but because of the cost and huge technical challenges of building capital ships. China had invested alot of money trying to buy a navy in the 1880s, but the next war they ended up in their fleet was destroyed.
However, in HOI3 terms the seven majors could easily be identified because they had more than 100 IC. China didn't. It was wrong to suggest this measure had no relevance - some of the AI scripts used this factor specifically to decide what actions a country should carry out. Majors built more special forces, planes and ships for instance.
It was a simple objective assessment of their industrial capability. Nothing to do with their contribution to the war, no indication of their fighting spirit or quality of their equipment, or leadership. And it's not a chicken/egg situation. They are not "major" because they have NF trees, they have trees because in 1936 they are majors.
France and Italy are major powers because they have far more industry than Poland, Sweden, China, etc. And that gives them the capability to produce battleships etc. Poland for instance could barely produce subs by 1939. France and Italy have colonies with resources and significant MP. And while the imperial powers of the Victorian era were in decline, France and Italy hadn't declined to the level say of Spain, which had lost nearly all of it's colonies and wealth by 1936.
China, Sweden, Poland, the Netherlands etc might be regarded as in a second tier of countries - if played very well, then there is a chance of making them into new major powers, leading their own factions and challenging the old majors.