This isn't really that relevant, all he had to do was disclose his connections. It's not the crime, it's the cover up. The US govt certainly understands hundreds if not thousands of academics have connections to China. It's not a crime to accept research money. It's lying about it that's the issue. If you're lying to the country about who you work for, what else are you lying about?
I don't think he would have remained undetected and the timing of all of this isn't that important. The recipients of the 1000 talents programs aren't some huge secret. It's actually very likely that the government knew all along and was seeing who would lie about it. If he had disclosed his connections, he wouldn't be facing charges.
It's not a crime to accept research money from China as an academic. It is a crime to accept money from China, lie to the government about it, then accept US govt defense dollars.
Imagine if you were working at AWS but moonlighting at Google Cloud. No one would bat an eye if you were fired and taken to court.