Which source is that? The official French report states the Malaysians matched the part by the maintenance seal which was still intact. Also the paint matches what was used on the original aircraft. Furthermore Boeing changed suppliers for the flaperon assembly to a company in Indian with the previous manufacturer coming from Spain. This also matches up with the flaperon being pre 2009.
The French won't 100% conclusively identify the part which is often misreported (usually on conspiracy theory websites ) as them saying it is not from the plane which is not the case. They are simply saying there is no 100% foolproof way of linking it to the accident aircraft but in all likelihood is the actual aircraft.
Here's the New York Times source;
http://www.nytimes.com/2015/08/06/world/asia/mh370-wing-reunion.html
...A person involved in the investigation said, however, that experts from Boeing and the National Transportation Safety Board who had seen the object, a piece of what is known as a flaperon, were not yet fully satisfied, and called for further analysis.
Their doubts were based on a modification to the flaperon part that did not appear to exactly match what they would expect from airline maintenance records, according to the person, who was not authorized to discuss the matter publicly.
French and Malaysian officials did not share the American hesitation, not least because no other Boeing 777 is unaccounted for....
About the missing ID plate
http://www.airlineratings.com/news/534/missing-id-plate-delays-confirmation-of-mh370
A critical identification plate on the Boeing 777 flaperon washed up on the Reunion Island that would tie it to MH370 is missing because of the effect of sea water on the adhesive that bonds it to the structure.
According to a former crash investigator the metal ID plate has almost certainly come away because of the “exposure to sea water.”
So what should have been a simple ID exercise now becomes a time consuming forensic investigation.
A Boeing part number (657BB) painted on to the flaperon confirms the object is from a Boeing 777, the Malaysian Deputy Transport Minister said yesterday.
“From the part number, it is confirmed that it is from a Boeing 777 aircraft. This information is from MAS (Malaysia Airlines). They have informed me,” Deputy Transport Minister Abdul Aziz Kaprawi said Friday. The flaperon has been sent to Toulouse in France to government laboratories, where the police, crash investigators and Boeing representatives expect to verify if it came from MH370.
Under French law air crashes are treated as criminal investigations and thus led by the police.
Malaysia Airlines MH370 disappeared on March 8 last year with 239 passengers and crew aboard on a flight from KL to Beijing.
Investigators will likely have to disassemble the flaperon to find other ID numbers that will link it to MH370...
Based on this second article, as soon as they take the flaperon apart they should know for sure, as the interior parts ought not to have been exposed to the elements like the missing ID plate was.