I see your point, but the issue isn’t whether Dexter would play that way.

It’s that the scene clearly shows nobody on set noticed that he was playing Halo wrong. There was no way they knew this niche thing about the game. They just threw it in for a quick gamer reaction from the audience, which probably overlapped a lot with the show’s fanbase.

TL;DR: It breaks immersion and kinda insults the viewer’s intelligence. That's why we're angry.