I'm hopeful this is a full on separate book (think like that Exodus sci-fi game WotC produced but didn't create) and not "just" a supplement, so it can adjust the 5.5e rules to be closer to old-school styles while using the current D&D "engine". No tieflings/aasimar/goliaths/dragonborn/orcs as playable races, for example, since that doesn't fit Greyhawk (no "weird" races, period, just standard things including Half-Elves and Half-Orcs). Maybe automatically use some of the optional rules like short rest being 1 day and long rest being 1 week, that kind of thing.
They won't really win back old-school fans who aren't happy with the current edition/tone unless they scale back some of the rules too IMHO. More than anything else, the issue is that most old-school players think the rules themselves don't lend themselves to a grittier style game, which is why they gravitate towards rules that emulate B/X or AD&D. If they don't fix that, this project is DOA.
I know one of the guys who said he's on Luke's team has been running a campaign in greyhawk for around 46 years, and he said there is no WotC internal personnel involved, so I'm hopeful they will be able to actually do things and not be forced to shoehorn in 5.5isms where they don't belong. He said they will absolutely be doing things like having inherently Chaotic Evil Orcs, and no fey Hobgoblins, stuff like that, so I'm hopeful about them removing a lot of the WotC changes that were pretty inexplicable.
This could be the reality of the "WotC is working on 6e and making it closer to old-school rules" rumor that's been floating around.