They said in the article they recreated it on their own. So it couldn’t just be the proof of concept.

This could mean they just put the files from the exploit on a drive and reproduced it. The author of the exploit claims it’s very complex and no one knows how it works yet.

Under cover/covert operations do actually happen.

So what’s the scenario they are protecting themselves against? Someone catching the agent right after they unlocked some encrypted drive with the USB drive still on them? It sounds very far fetched to me that FBI would request a backdroor from Microsoft with this very specific requirement. I think it’s more likely they would cover it on their side with some easily erasable USB drive. Plus such a solution would also let them get rid of the backdoor if they are caught before they used it.