Ugh, Jace seriously only has one brain cell to rub together! Clearly, Jace and Clary (who is equally dumb) are meant to be! I’m serious guys, anyone with a working brain cells would have immediately figured out that this wasn’t Clary.
Clary was at Luke’s house, and there is no way she could have made it there before Jace without convincing Magnus to open a portal for her. And as we have previously seen, Magnus wouldn’t have lifted a finger without being paid it, because Magnus continues to be a mercenary jackass.
So by process of logical deduction, the person we see here can’t be Clary. Yet, Jace never once stops to question that this is an illusion or a trap by a demon that can shapeshift . So inclusion: Jace is once again being an utter and complete dumbass!