I don't think we've attempted to study if rats have internal monolgues all that much, yet. It wouldn't surprise me if they did, or did not. I wouldn't say it is safe to assume they don't.

About the only real animal model has shown that some species of monkey probably do. [0]

[0] https://www.sciencedirect.com/science/article/abs/pii/S01664...

On 4chan and reddit, they actually believe that those without an "inner monologue" have no soul.

Reddit has roughly 400 million weekly users. You think all of them believe the same thing?

That's what the evidence suggests.

Just to underpin how wrong this comment is, I - the person who posted the medical explanation - have been rather active on both those platforms.