That makes no sense to me. If the "setting" of Saltmarsh is open for DMsGuild publication, and Saltmarsh is described as being on the coast of Keoland (as it should be,) then how is Keoland itself not going to be open, with the pantheons of Oerth, the Citadel of Eight, etc.?
I'm more than a bit tired of whatever game they're playing with Greyhawk, and I'm still salty over the fact that you can publish and adventure for D&D set in MtG but not in the original D&D published setting (or the 2nd one, for that matter, which is "The Known World" now called Mystara.) You've already got people creating content for these settings "for free" for those who support them on Patreon, as well as for Dark Sun and Dragonlance. If WotC wants to take a cut of products made for those settings, I think they need to get on it.
So, obviously, I don't actually know what they're doing. But the fact that they are keeping Greyhawk back might be seen as an encouraging sign that they intend to provide more robust support for the setting and are waiting to make it available on DMsG so they can time it with the release of that additional support.
Usually, their planning horizons are pretty long, so that might mean it'll be awhile though.
But, heck, who knows?
AD