There aren't any other missing 777's, so all they really need is to conclusively determine the part came from a 777 and process of elimination takes care of the rest.
Still, I understand them wanting to be extremely cautious.
To be definitive, though, you need to find evidence that proves that the piece you found was present on the aircraft for the flight where it went missing. You have to have some way to say, "Ah, yes, this flaperon is a Boeing part number, with serial number #XXXXXX. Maintenance records show that this exact serial number was installed on the aircraft in question, and its configuration as found matches the configuration of record for this part on the aircraft in question." Then you know for sure to a degree that will stand up in any inquiry.
Well, not quite, right? If you wanna be paranoid, it could be a maintenance/replacement part from another 777, deliberately planted. (Not that I believe that's likely, just saying.)
Still, I understand them wanting to be extremely cautious.