I can't find who's the source on this. The best I can find is "people familiar with the investigation", but both the FBI and Cellebrite refused to comment on the story. This article quotes Bloomberg, which seems to be a copy of the Washington Post's article without all of the fluff.

I'm not surprised, there was a recent report that showed that Cellebrite can unlock any phone except for recent iOS and GrapheneOS. I'm just confused who "the people" that are being quoted everywhere are supposed to be.