I'm always a bit surprised it is always professors being targeted. Aren't a more likely source of leaks... grad students or undergrads? Or hell, I'd think janitors would be the best vector.

The prof's expertise is often no longer in the subject, but rather in leading a team of people in the subject and leading people in how to write about the subject. The two main advantages is that they are a constant contact and they are a key decider in who gets accepted[0]. The grad students do all the work (often including grant writing) and are going to be up to date on information. Mistakes or slip-ups are much more easily overlooked and best of all, you can transition them out in 2, 4, or up to 8 years depending on their success and suspicion level. They also have broad access to materials, are often left in the lab alone for hours, and do not rise suspicion if they go into the lab at 3am. Undergrads have similar advantages but don't have as broad access (typically) and might arise suspicion at going into the lab at 3am, but do have an advantage in being able to pass things off as dumb mistakes because everyone holds them in such low regards. And of course, janitors have all those advantages but also have keys to all the labs and offices where materials are routinely left out in the open "because the door is locked."

Of course, the best way to do knowledge transfer is to just send your civilians to the country to get educated and then bring them back. Deng's opening up of China was a major success. The fear being that they couldn't import western education without western idealologies but that was super successful.

That said, one time I was working on an ITAR project and it had to deal with two restricted technologies: nuclear and nanomaterials. This was while I was working for an aerospace company[1] and the two other domain teams were at a uni. The nuclear team's lead student was Iranian (had a green card) and the nano-team was all Chinese. It was a really interesting situation because I could talk in full with the 2 professors but anytime I had to talk to students I couldn't tell them things despite the fact that we all knew each other knew what was going on. It wouldn't require anyone saying anything directly to figure this out.

We also had the FBI visit (that's not uncommon when you do ITAR work) and they asked me to "keep an eye out" for certain things. So I guess I was a spy? (lol) They were convinced that the policy would ensure that there was no information leakage and that the nuclear team couldn't figure out what the nano team was doing, vise versa, and that neither could figure out the end goal. Then again the agents tried to convince me that they routinely break PGP encryption and that TOR is exclusively used by terrorists and drug dealers. So... I guess all this is to say that I'm not all that surprised that baseless accusations arise. The only thing that is surprising is how people can believe in a complex deep state because the truth is that government works just like your company: a bureaucratic shitshow.[2]

[0] Take for example a professor at my university. He is <south-east asian>-ian (i.e. Not Chinese/Korean/Japanese/Indian). He has 4 students and has graduated another ~5. All 4 are from the same country as he is, and 2 of the alumni are also from that country. Those 2 were admitted into the grad program by him, 2 of the other 3 were students who switched professors later on (I don't know the 3rd one), and 2/3 of them are from a neighbor country. I'm not saying these people don't deserve to be here, that they don't have the qualifications, or any of that. They do! I have no doubt about that. They were/are all great people and have done well and had good credentials. But if we're being honest here, this isn't an unbiased process. The truth is that there are a ton of equally qualified candidates to select from and when you have to make those decisions you rely on much more subjective information like your social networks and it's probably unsurprising that those strongly correlate with ethnicity (not to mention factors like "culture fit" and the fact that because the composition the whole group speaks in their native tongue which enables them to communicate better but unfortunately disadvantages anyone who doesn't. And weirdly enough, switching to English would disadvantage the group as a whole because who isn't more proficient in their native language?)

[1] This is all I can say but I'm sure plenty of you know EXACTLY what I was working on due to the intersection. In fact, some of the info is public but I'm told I still shouldn't talk.

[2] If your company doesn't work like this, well it probably will when it grows.