I would have thought this particular type of item is not regarded as a consumable in the logistic chain.
As such even without serial numbers they should be able to tie down all the ones manufactured, and hence where they are in the logistics chain. Whether they be in stock somewhere, or flying around.
Another words Boeing has manufactured x and we have accounted for Y. If they are short the number on the aircraft only, then clearly its what we are looking for. Collating that data may take a bit hence its possible to be a bit coy until that has been done.
Accounting for the flaperons which are currently fitted to aircraft currently is the easy bit. There will no doubt be many test specimens, manufacturing rejects, flown but replaced components, exhibition display components, the list goes on. Accounting for the current wherebouts or full destruction of each will be a painstaking task. It only takes one flaperon to have gone to a recycler who didn't do what they were contracted to with it, and there is an element of doubt to the identity the component found on Reunion.
I do find it somewhat surprising that there are not serial numbers on the constituent parts of the found flaperon; although from my own experience, I'm aware that not every aerospace company's traceability is as good as a quality controller (or accident investigator) may wish. Often traceability to a specific batch is deemed sufficient.