We do not even know if the FBI knows the vulnerability. The phone could have been attacked by a contractor working for the FBI without the contractor sharing the tools.

This is a great point. Cellebrite's known for their black box approach. Law enforcements and Courts can send their locked iPhone and get it back unlocked. They'll just get the job done without knowing what was actually performed on the phone. This might as well be the case here.

Doesn't this approach cast doubt on the legitimacy of any evidence obtained from the device?

Don't think so, it's basically like lock picking a suspect's apartment with a warrant. What is found should be valid evidence, no? Law enforcement can break the door or call a locksmith and pay him for his service. In this case Cellebrite's the locksmith.

That seems reasonable, but usually investigators would be on the scene while the locksmith works. If the process is a black box that happens while the device is in Cellebrite's possession isn't it more like calling up the locksmith from the precinct and saying "hey, could you open up the apartment at this address and call us when you're done? Don't go inside or touch anything though, thanks!" What prevents Cellebrite employees from planting or deleting evidence after the device is unlocked and before returning it? What guarantees are there that Cellebrite's unlocking process doesn't intentionally or unintentionally modify some other aspect of the device? Scouts honor? What if their process is to just flash a new rom filled with child porn and no passcode then skip merrily to the bank to cash their checks?

Yep, I understand that. I don't think that was posed as a problem in the Italian case I know of, although I think that in the U.S. rules on the chain of custody are stricter, indeed. That's a good point.