It gets thrown around a lot because it is not only correct, but makes sense. The whole notion that Islam is evaluated by its extremists’ beliefs is as ridiculous for that faith as it is for any other. It also enables extremism and undermines efforts to combat them.
The history of Christianity is filled with extreme violence between Christians and every other faith it has dealt with and within its own sects. All done in the name of Jesus (or their version of him). Reading the Bible would never tell you that genocide and sectarian violence has been inherent to Christian religious practice from the practically its outset.
If one evaluated Judaism on the basis of the acts of its patriarchs and readings of the Old Testament you would believe they are also somehow extremely violent and genocidal.
But doing so would be not only incorrect, but extremely lazy. Yet doing so for Islam somehow is different? Of course not.
You are merely trading in generalizations for a group of over a billion people. There is no way something like that can be done with a straight face.