Firstly, that statement is true of monotheistic faiths. Some faiths are polytheistic, and can deal with the idea of many gods.
Secondly, this whole attitude of focusing on the bit of their belief that you disagree with is actually the source of religious conflict. Yes, these dudes believe in a bunch of different gods that maybe you do or don’t believe in. Can you look away from that? How about looking at the fact that they’ve all come together to ask blessings / well-wishes / good things for the race coming up?
Religions come into conflict when their adherents focus on their differences. He’ll, religions come into internal conflict when they focus on their internal differences. See Shi’ite/Sunni conflicts, Catholic/Protestant conflicts. An atheist might claim to be above such petty differences, but when I see a person zoom straight in on religious differences, it looks a lot like religious fundamentalism to me. I see the same proud “I know what’s right, all those other morons are wrong” kind of attitude.
So even if your remark was a joke, off-hand thing, could we please not? No need to encourage that kind of thing.