They mention the case of Charles Leiber [1] which might be informative.
[1] https://en.wikipedia.org/wiki/Charles_M._Lieber#Federal_arre...
[1] https://en.wikipedia.org/wiki/Charles_M._Lieber#Federal_arre...
I wonder what other evidence they have here, eg him actually receiving funds from them ... if a Chinese university sends you a recruiting email and it goes to your spam folder, and then you say you have no affiliation with them, can you be indicted for that based on an "intercepted email"? One would hope not!
I suspect they had evidence to indicate he was paid, but couldn't produce specific financial records to prove it without a doubt. In cases like this, the FBI typically goes for the jugular by pursuing espionage charges. This can be difficult because it often requires concrete evidence of financial gain from the foreign entity explicitly for the information provided or actions taken. The mere appearance of financial gain is not enough, they'd want to see the literal check stubs and account statements.
[1] https://www.justice.gov/opa/pr/harvard-university-professor-...