I believe he means "the use of asterisks to mark required fields" was invented for the web. Otherwise, you'd have to be quite young and quite sheltered to not have seen asterisks in places older than the web.
It's hardly the same thing. A footnote is used for any relevant information that doesn't belong for whatever reason in the body of the text. That it could say "Required" seems neither here nor there.
Yes, and the one on paper forms almost always was used for "required".
Besides, when it didn't, normally there were many of them, so they were numbered instead of an asterisk. The asterisk itself was all but reserved for "required".