This was already in the base rules (top of page 10, Mythos phase, part C3):
Exception: If there are five or more investigators playing, then draw and place two monsters instead of one.
The Dunwich rules are just saying explicitly that you also do this on the first mythos card of the game. That was already true in the base rules, since they said to do it as described above:
Finally, the first player draws the top card of the mythos deck and resolves it as described in the Mythos Phase section of the rules.
It's just more explicit in Dunwich, in the same spirit as all the FAQ provided at the end of the rules.