Starting as soon as 1933, the Chinese (National) Army was trained by the Germans. The Chinese even used Panzer I !
It would have made sense to put them in any "doctrine" :
- Pan Asia because well, Asia
- Axis because they were trained and organized along the German doctrine, until at least the Battle of Nanking where they lost all their German-trained divisions
- USSR because they had the same case of "mass army" and reliance on mass & militia
- Allies because well, that's the side they ended the war in.
I believe Bytro did not put them Pan Asia so Asia is a bit more diversified in terms of doctrine, and did not want to put allies because it is already the most common doctrine (and it includes its relative neighbours India, French Indochina and Burma).
That leaves Comintern, but it would be strange to see Nationalist China, which fought Communists until the end, be "Comintern", so well Axis doctrine it is.
I still believe in terms of "stat bonus" it should have been Komintern but since narratively it makes no sense I believe should have been Pan Asia (terrain dominance and "light" unit bonus would fit better than having great motorized infantry and expensive-but-powerful units) but I understand where they come from.

