One thing about the history of that game that they didn't mention in the film.... That year the championship was determined by a many-match round robin. By the time of the match with Ponzi, Mosconi had already locked up the championship by many games. That little detail makes the final shot believable.
I don't know. Willie hated to lose. I can't imagine him being willing to lose a match even when he locked up the title. But either way, this is NOT how it was portrayed in the film clip. Once they demonstrate they are willing to stretch the truth a little the rest is pure speculation.