According to M-W, ‘infidel’ means
1 : one who is not a Christian or who opposes Christianity
2 a : an unbeliever with respect to a particular religion b : one who acknowledges no religious belief
3 : a disbeliever in something specified or understood
It seems to me that definition 2a fits the attackers quite nicely.
I’m not 100% positive, but I believe this is the same mosque that 101st Airborne soldiers did not return fire into (so as not to offend the locals) when they were attacked from within it in April. None of the coverage I’ve read indicates that it is, so maybe it’s a different mosque in Najaf. I’m on my own so far on that point.
Either way, I don’t see how this is going to increase support for the anti-US front.