> While threats from the Chinese government may be valid, it's more likely that they don't want to / cannot risk their H-1B status or green card priority date. So their immediate threat is more likely (indirectly) coming from USG.

That doesn't make a lot of sense. I don't see why the USG would want to sanction them, especially since this letter has nothing to do with the USG.

If they're discouraged from signing due to their US visa status, the mechanism that makes more sense is that they fear Google could fire them and it would take them longer than 60 days to line up another job that could sponsor them.

What is the risk, that their status would be changed in response to their involvement? If so, would there be an officially articulated basis?

They can get fired. If they're still an H-1B, that basically requires them to leave the country immediately.

There's a short grace period in practice (usually, and this isn't official) if you have another job all lined up that is ready to sponsor you. But if you were an H-1B applying for a green card via your employer, this basically resets your position in line, unless you're in late stages of the process. And keep in mind that the wait is measured in years for many countries (e.g. for India, >9 years right now unless you're in the "exceptional" category).