So, would that also include the max hand size (decreased to 15?)
Possibly, but not as far as I know.
Also: Is this official?
I'd say yes for now. "Reduce opponent's hand" doesn't really have an official definition AFAIK, but it has been understood for the past 13? years since Warriors came out to mean what my suggested play as said. However, it would probably be updated to this given the new usage of restrict abilities:
"Restrict opponent from having more than 7 cards in hand (or 9 if Tables of the Law is active) at the end of each of his turns, until this character is discarded."
In any case, I can be fairly certain that at Nationals or any future tournament, D&H will not force your opponent to discard any cards above 7, nor will it stop opponents from drawing beyond 7 cards. So hopefully none of you are expecting to build decks based on that.