its not an assumption
they mostl likely all are.
you dont get out of china if you are not good with the ccp. they routinely have students spy for them as part of the deal socthey can go abroad.
its been well known since the 1990s and probably before that.
I wonder if it ever happened that one of these Chinese exchange students ended up in an economics course and got into an argument with one of the professors about what Marx really thought on a particular topic.