They're saying "There ought to be no gods other than the one I believe in", despite the fact that other people believe in other gods. They think that those people are delusional and believe in a god that isn't there, but that they're perfectly reasonable to believe in theirs. They think it's absolutely absurd to think that Lord Vishnu had a flower growing out of his navel which he separated into three parts, creating the earth from one of them. But, they think it's perfectly reasonable that Elohim created the heavens and the earth in six days.

Not only that, but they don't even believe that this "Lord Vishnu" exists. It's not that the Hindus got the story wrong and that he was just standing off to the side while Elohim did the work, they think that Hindus are suckers for thinking that he even exists, and that it's only their god that exists.

If there's a presumed need for a deity to exist to explain the world (which is absurd), then why restrict it to just one deity? Many believers throughout time have believed that there are many gods, just that theirs are the strongest. But, modern monotheists somehow believe that it's a fantasy that other gods exist, but not that theirs exists.