People's belief in something is evidence for that thing in the sense that in general it's more likely for people to believe in a thing if it's true. Less Wrongers sometimes use the phrase "Bayesian evidence" when they want to explicitly include this type of evidence that is excluded by other standards of evidence.

One way to think about this: Imagine that there are a bunch of parallel universes, some of which have a flat Earth and some of which have a spherical Earth, and you don't know which type of universe you're in. If you look around and see that a bunch of people believe the Earth is flat, you should judge it as more likely you're in a flat-Earth universe than if you looked around and saw few or no flat-Earthers.

However, people's beliefs are often weak evidence that can be outweighed by other evidence. The fact that many people believe in a god is evidence that there is a god, but (I think) it's outweighed by other evidence that there is not a god.

See also "Argument Screens off Authority".