Islam is a monotheistic religion, it holds that Allah is the same as the God in at least two other religions, Judaism and Christianity. There’s no reason why it might not be more, Mohammed just didn’t and couldn’t get around to looking at all the other religions.
Where you’re going wrong here is in assuming there’s a right religion. There’s not, there are just different ways of being spiritual and religious. It’s all the same ‘stuff’ in the end, just different ways of getting there.