I think this is a bit of an extreme response. I don't think the parent was suggesting that every Chinese PhD student in the US (or even a majority of them) is a CCP agent, just that it does happen, and the more Chinese PhD students who end up in the US, it's likely that more of them will be beholden to the CCP in some way.

I think your framing as "agents" is a bit sensationalized. You're evoking images of these highly-trained spies willing to do anything to steal for their country. My expectation is that most folks are just regular-Joe type people who just want a good education, but are pressured by their government to pass back certain kinds of information (with under-specified by scary consequences for their family if they don't comply). I have a lot of sympathy for the people who get caught up in this situation. They're people who are trapped in a way that requires them to pay a higher (moral) price to obtain a good education. I'm not sure I'd have the strength to behave any differently if I were in their shoes.

(Also consider that the US's industrialization was fueled by plenty of academic and industrial espionage. That doesn't make what goes on today ok, but let's not pretend our historical hands are clean.)

America is many times less racist than China is. I think it is proper to debate the degree to which allowing Chinese citizens to immigrate helps or hurts America.