That is probably the strongest evidence I could see the FBI being in specific possession of.I wonder whether the FBI has some other information based on informants from China that the Chinese were up to something in that lab? It would be the kind of intelligence that scientists would not be able to detect in their research; more like a police investigation?
The satellite idea doesn't sound very persuasive to me and the FBI is pretty famously bad at its biological analysis (remember Amerithrax?).
But as a law enforcement agency, having informants who are giving them testimony about something specific would definitely be something they would have that outside researchers wouldn't.
All you need is a couple of them who aren't really all that trustworthy and you can get to "moderate confidence". They would have good reason to believe it, but the informant isn't unassailable and they don't have a way to confirm it. That would fit right in line.
Of course, they aren't telling us what they do have, so that's just wild speculation. But it is a guess that actually sounds pretty reasonable as a possibility.