Because you seem quite intent on trying to prove something which I think you're mistaken about i.e. that Gonloopy stated that diacetyl can cause popcorn lung, when in fact the quote you used said "potentially can". If you are correct in your assertion that nothing has been proved, then it stands to reason that nothing has been disproved either. Therefore, diacetyl potentially can cause popcorn lung but it also potentially can't if your assertion is correct. It would seem that the matter is still up for debate and debate can be healthy, as long as it doesn't turn into some kind of redundant point scoring exercise.