I think what they mean is if you own Reach digitally, will you be able to download the Halo Reach Campaign/ Firefight add-on for MCC for free?I'd put the probability for that at very little.
However you will get the Reach premium edition content as part of Gamepass.
Probably not worth it if it's the only thing you'll ever play on Gamepass, but there's a lot to be said for it as a package: case in point - since its launch I've probably played more Sea of Thieves than any other game, but never paid a penny for it because it's on Gamepass. I've tried out games I'd never have otherwise e.g. Hellblade: Senua's Sacrifice, Abzu, Shadow of the Tomb Raider, Gears of War 4. I'll also be able to play Infinite on day 1 without spending a penny on it if I wanted to. Just depends what kind of gamer you are: if you only play a few titles then perhaps individual content is right for you: if not then Gamepass is a pretty sweet deal. I used to be in the former category, but I'm definitely a convert now.