If I recall correctly you could change difficulty during a TW2 playthrough too. Can't remember how Dark items got handled though.
You could change FROM dark mode to another one at any time, but lost the exclusive dark mode items. You couldn't change TO dark mode. The other modes could be switched as and when you wanted.
And yes, unless he just didn't explain it well, it would imply that there aren't dark mode items this time, that it's just a difficulty level.