Every quarterback is a system quarterback to some degree or another.
Each team has its own system regardless of whether they drafted a quarterback they believed fit the system they had installed, or they built said system to accommodate the individual talents of their quarterback (like, say, the Ravens).
Hell, Joe Montana had Bill Walsh as a coach and Jerry Rice to throw to, but I don't ever remember anyone calling him a "system" quarterback. To me, it's largely a myth.
I just mean it as the last handful of years Brady has gotten by on dumpoffs to guys in open space, and Gronk being a monster. We haven’t seen him on another team, unlike guys like Manning and Favre, Montana to a degree, who went elsewhere and elevated the guys around them regardless of the head coach.
Arians likes to air it out a lot, and deep. If Brady goes there and throws 40+ times a game all of a sudden, and the results are poor...well, it doesn’t really matter. It just means other guys have been bigger risk takers with the ball and more successful. Manning, Favre, Brees in that conversation.