This doesn't work like a Sort type, but more like a Limit type. I would expect it to sort by time.

Yes, that's basically exactly how it works. It's supposed to show you “the top posts in the last hour”, so I think sorting by score does make sense here.

(In fact, the time range is actually being separated from the “top” sort type in lemmy 1.0.0, so it should be more intuitive then. There'll just be a “top” sort order, alongside a filter for post time.)

It doesn't work on "Overview" or "Comments", only if I limit to "Posts". So yes, it works on posts, but only if no comments are involved by overview.

Yup. That part definitely looks like a bug.