You're not entirely wrong, but I don't think they are entirely orthogonal either.

There is almost certainly a significant overlap between introversion and social anxiety/dread, even if they aren't 1:1 related

If nothing else, many people with a lot of social anxiety will claim they are simply introverts in order to cover for their anxiety

The fear of reaching the point of social exhaustion, as introverts quickly do, is the anxiety.