This sounds like a threat to me that all black players will kneel or do something else disrespectful. Then it would be on the owners as how to respond. Suppose no disrespectful player plays. There might not be enough players to play the game, so the game is forfeited and the fans leave the stadium PISSED.
Since the threat is league-wide, this could happen in multiple cities.
So, I don't put the percentage very high but I would give it a 5% chance of happening.
They got 53 guys on the roster about 3/4 don't make shit in NFL terms. They will play both ways. I put the chance @ less than 0.