Presuming this is a genuine question: the difference is whether one goes on to do more work after the 'guard' return, or whether one is just choosing which value to return.

I think the style guide is being silly, and there is no simple rule that says "always use an else" or "never use an else". I'd see "Avoid unnecessary indentation" as a better rule.

One's an early return due to error, one's a conditional return value.