I wouldn’t call myself any kind of official on the rules, first off.
Yeah, this is a bit different than the other nations. I would say first off that this is one of the bigger differences between versions 2 and 3. I believe the game makers went out intentionally with the idea of making conflict between the Comintern and Allies a more realistic possibility. Example, in Version 2, the Allies and Comintern couldn’t declare war on each other until the Axis had been defeated (though I think there’s a little loophole in the Monroe Doctrine there). Same with China, the KMT or CCP would not have been allowed to declare war on the Comintern or Allies, respectively, until all foreign powers were expelled from mainland China.
The Version 3 rules open this up for conflict earlier on, so I think this was absolutely intentional on their end.
Of note, in order for the KMT or CCP to get Major Power status, they need to effectively have control of all of China. They need 13 IPP, I think there are only like 16 IPP possible in all of China. So while not necessarily needing FULL control of all China, that IPP threshold probably effectively means they’ve won the Chinese Civil War, and arguably have expelled Japanese power on the mainland for that to have happened also.
That’s my long way of saying that their ability to declare war is probably not as easy as it reads on the surface. Plus, I think the KMT would have a hard time deciding to declare war on the USSR unilaterally until the other Allied nations were available to assist!