I think the point is that it's a cop out to decide "Oh, those guys aren't technically Muslims because they do things I don't approve of" when their actions are based on the same (and what some people would consider to be harmful) belief system.
In fact, I'm sure a lot of these extremists consider themselves to be the only "true" Muslims. I'm not arguing that they're right. I'm saying that ideologies are so widely varied within a single religion that it's almost arbitrary to say "Oh, that person isn't a real Christian because she got an abortion", or "That person isn't a real Jew because he isn't Orthodox". There's always going to be some sect or group that's going to make that point about others who identify as the same religion as them, so who even gets to make that call? Not to mention, it comes off as a way to waive any responsibility for or even acknowledge the negative aspects of those religions and their consequences.