For those arguing the statement that 'ISIS' are not muslims', your explanations are absolutely corrrect, but you don't understand the person's statement.
It is true that they are muslims. However, why would ISIS follow their own extremist teachings? That's not Islam. A true muslim does not follow the way they are now. That's why they're not considered proper muslims.
Generally, even if people use muslim as an 'identity' and not follow their teachings correctly, they're still not proper muslims. Despite this, we can't judge them, so let god do that and guide them, including other people who can still help them.