I agree and disagree with him at the same time. I agree that a new player starting from zero gives great experience, but I disagree about having that happen within an established and-or higher-level party as the experience won't be nearly as good: either the existing characters will cover for the new one until it catches up, or the new one will drop dead from encounters which the others can easily survive.
You've misremembered and it seems like you've misread me too?
Gary suggests that new players start off at 1st, and he specifically suggests that if you've got an ongoing campaign that they should adventure separately from higher level PCs.
Personally, I don't mind letting them join a higher level party, at least if I'm not running an open table where I have time to run separate low level sessions. If I DO have time to run low level sessions on top of the higher level ones, definitely, great to just do that. But if I have to run them together that's still ok, especially because they'll catch up really fast using by the book 1E experience rules.
I think 5e has dialled it back some from 4e - the gap's not as big.
That's largely true, though 5E characters still got more forgiving rules for death and dying than 4E had, and of course 5E is designed to rush you through the first two levels in a couple of sessions, so you're rapidly to 3rd, which is a similar (or even greater, relative to enemies) power level as a 1st level 4E character.
And at the same time and for the same reason - fatigue - one's ability to deal out damage also becomes worse. The end result would be pretty much a wash, hm?
Generally I'd say not really, no. It takes very little force to pierce a human being or fracture their skull with a weapon. If you've still got enough energy to swing the thing at all you've still got the capability to kill someone with it. You COULD say it's a wash, but I don't think it really is, so I think there's at least a reasonable argument to be made that Into the Odd or Worlds Without Number's systems, with their automatic damage, are at least as simulationist as D&D.