Not all trumpet players can play forward jaw. So the statement is impossible.If it were the case, all lead players would play upstream and all "legit" downstream.
Well it is LIMITED by the jaw positioning most of the time. The forward jaw player usually has his upper lip limited in motion. can not penetrate the mouthpiece as far. Situated like this it can not emit the full tonal spectrum. A fact that drives some upstream cats nuts.The sound is not predetermined by jaw position or "up/downstream"..
As high as he can usually play the forwards jaw trumpet player can not get a tone as big as Maynards.