It is the attribution of that extremism to being a part of 'muhammad's fanbase' that is islamaphobic, not pointing to an example of the extremism itself.

A member of a group committing murder and citing that group's beliefs isn't a justification for casually implying that members of that group are murderous, even if it's true is the most limited sense of the word.

Especially when that group is itself subject to extreme violence and genocide on the basis of their membership.