Это делает его пререквизитом, но не обязательно unusual background'ом. Точно так же, как никто не называет

Er, Trained By A Master doesn't include an Unusual Background... it is an Unusual Background. What's the background? You were trained by a master, of course. That's how it was defined when it first appeared in the game, and that's how I still regard it as a designer. The fact that it has game-mechanical benefits doesn't change this. An Unusual Background has to provide concrete benefits or it isn't a proper Unusual Background — read the advantage description.