Here's the thing, though: it's not a contract. It is, according to that tweet, something in the "original Realms agreement." Presumably that means the purchase agreement for TSR buying the IP rights to the Forgotten Realms. In which case, even if there's a clause which means exactly what Greenwood says it means (which isn't to say he's a liar; people get the specifics of legal agreements wrong all the time), then the validity of it is still in doubt simply because it's not something WotC has to acknowledge or honor. The purchase of the rights won't be invalidated if they somehow come out and say "no, whatever Ed Greenwood writes is not inherently canon, regardless of whether or not we contradict it in a published product."
Of course, a lot of this concerns the nature of what "canon" is (and why so many fans consider it important), but that's a separate discussion.
So... here's a thing I learned in Real Estate:
When you sell something, you can sell it conditionally, and you can sell it piecemeal.
If I own a home I can sell the prospecting rights under the house without selling the home. I can sell the airspace rights above it without selling the home. And I can sell off portions of the property as small as 1 square foot without selling the home. Ostensibly, I could also sell specific rooms of the house while retaining use rights to the rest of the house, up to and including ingress/egress easements. I could even sell every part of the house except for one bedroom, and use an ingress-egress easement to pass through the parts of the house I don't own on my way in and out of the house.
And I can retain rights for conditionality purposes. Such as selling my home at a -distinctly- lower price than other homes in the area to provide it to a halfway house program or for church use or other options, while retaining the exclusive legal right to drink alcohol on the property. And if someone in that halfway house or church property drinks booze on the premises, I can reclaim the entire property while keeping the money.
Granted. IP rights aren't exactly real estate rights. But. We see some of those same functions showing up in Hollywood Movies, too. For example the Fantastic Four's film rights were owned by Constantin Films after they bought them in 1986.
And in order to try and retain the rights for longer, they produced a really crappy low-budget Fantastic 4 movie with paid actors and crew that was never commercially released. It worked. Because the contract said they had to MAKE a Fantastic 4 film, not distribute it.
Why did they have to make the trash version? Bernd Eichinger bought the rights in order to do a large scale production of the Fantastic 4, but securing funding, finding a decent script, and the like took longer than anticipated and the rights were about to revert.
He'd later go on and make the 2005 Fantastic 4 film by sublicensing to Fox Searchlight in order to get the funding needed. And then when the rights were about to revert, AGAIN, Bernd did the 2015 version.
Constantin Films bought the rights on the condition that they made a film within 10 years. So every 10 years or so (1994, 2005, 2015, 2025) there's been another movie. Disney purchased Fox Studios, and the Sublicensed rights in the process. And bought out Constantin's rights to produce Fantastic 4 movies along the way. So we'll definitely see another Fantastic 4 film post-First Steps by 2035. And every 10 years after that, too, just so Disney retains the rights.
So it's not ENTIRELY out of the question that he 'Retained Rights' to continue to make authoritative statements about the canon of the Forgotten Realms until such time as he is directly contradicted by TSR. And since WotC can only buy the rights that TSR possesses, they cannot buy the right he retained.
I think he knows the purchase agreement better than we do, though.
