We should not be happy that it "most probably is"
In complete agreement there, this is what I said with different words. The issue at hand is that it might not be possible to make a direct link with MSN 28420, as the S/N placard is missing and the rest of the structure likely has no unique identifiers.
In absence of that, an equal proof is to account for all other such components ever built (and probably the range can be narrowed down to a specific manufacturing period). For the ones in service since airframe manufacture this is relatively easy, Boeing has them on file. It is the ones not installed that will be difficult to trace, dozens if not hundreds of withdrawal from service records will need to be collected from MRO stations around the world, then the found flaperon must be compared against the scrapping method used to eliminate each. This will take months (if not more) and easily explains the lack of further communication from the French investigators.