With all the corners that get cut in the business of running a modern martial arts school and keeping it financially afloat, I still say it is proof of nothing. People are often tempted to violate their already low-to-mediocre standards if it keeps the customers happy and keeps them paying the membership fees.
Proof is too strong a word in any event. “Evidence suggesting” would be more appropriate. Nothing is proof except on a case-by-case basis after the fact. “Proof that he can fight”? No. At best, there is proof that he won a fight in the past. “Evidence suggesting that he can fight”? Ok, I can accept that, assuming the instructor holds high standards.