Why wouldn't he be? All the players you named (except for Twistzz, who plays a supportive role) play frag-heavy roles, where they are expected to be the topfraggers.
Stewie on the other hand plays entry and anchor, roles which often have very few frags. It's not their job to drop multikills every round. At best he's supposed to bring in 1 frag per round, and in some rounds his only role is to throw some util and stay alive for info. They are not roles that will give a lot of frags.
I'm not necessarily saying that he is as good as ELiGE or Brehze when it comes to stats, but if he played the same positions as they do, he would at the very least be a lot closer.
EDIT: TL:DR - I'm not 100% sure he's as good/better than those players, but there's also not much to suggest that he would be worse.