Arthur's yanking around of his robe does at least tug John off-balance, and so for the moment he's preoccupied with counter-struggling against Arthur's struggling. It does at least draw this stalled moment out enough for John to notice the weapon the other man named John has drawn.

"Take a piece out of- roommate?!" John (in the mask) begins, stops short, and looks down at Arthur.

The man has a weapon and his thoughts should be turned towards disarming him, getting himself and Arthur to safety, but instead what comes out of his mouth is, "Arthur, what the fuck is he talking about?"