The “No True Scotsman fallacy” goes like this:
Smith: All Scotsmen are loyal and brave.
Jones: But McDougal over there is a Scotsman, and he was arrested by his commanding officer for running from the enemy.
Smith: Well, if that’s right, it just shows that McDougal wasn’t a TRUE Scotsman.
Just as convention about racism and sexism is supported ultimately by a variation on the fallacious appeal to consequences–if racial or sexual variation in behavior and aptitudes were real it would be bad (or lead to bad things), therefore it (or race itself) does not exist–so too is the “not all Muslims” reflex something that any thinking person, regardless of opinion, should reject.
As one famous Muslim said:
Social justice warriors: all the intellectual depth of a Muhammad Ali, without the humor.