That's straight B.S. from Greenwood.
He's trying to claim every story he ever wrote, including crap from the 3rd grade, as part of the Forgotten Realms. AT BEST he claim 1979 with his first Dragon publishing, and that is shaky.
FR was first published in '87, D&D had kicked-off in Gygax's basement by '72. Greenwood was just 13.
Oh yeah, it comes off as the grumblings of an old man who is tied to something great but didn't end up being great himself. I found his Elminster novels insufferable, and his Youtube channel (which I have to listen to for research purposes) to sound like someone trying to claim glory on the back of people who did the real heavy lifting for his open-ended world.
The problem is, it's tough to falsify what he's talking about, particularly in the context of D&D. D&D's storytelling mode/world creation tends to be incremental - campaign worlds start off as a dungeon, or a single campaign, and as the campaign expands, as more work is put into the backstory, it may eventually flesh out to be a comprehensive world. Hell, that's pretty much the story of how Castle Greyhawk became the Greyhawk campaign setting. So, if D&D in general accepts the "this little snippet might eventually become a world," it's hard to tell the guy who came up with Forgotten Realms that the stories he wrote early on weren't an origin of the world when he claims it is. A great example of this is something something like The Known World/Mystara, which was the standard D&D setting for a time. Some of the D&D devs at the time had been running the campaign setting for a long time and then suddenly it became the central setting for D&D. Incremental creation for years before it became a big deal - The Known World was around as a fan creation long before it became an official D&D property in 1981 or so.
The Known, Hollow World - Mystara - Yawning Portal
Honestly, shit like this is why D&D is an interesting object for research. Many of the central fictional settings of D&D didn't even start to be marketed - they started as the brainchildren of fans, long before they were part of the corporate side of the game. In a world where fan fiction turns into billion dollar franchises, the type of incremental, decentralized storytelling that D&D has been doing since the 70's is neat to look back on.
That being said, the reality is that FR's content has gone so, so far beyond anything Greenwood came up with that it's like giving the person who did the kickoff credit for everything that happened in the football game. Whatever is notable and good about FR, if there is anything (it's a versatile, generic, fantasy world with lots of room for expansion), Greenwood is a footnote in its history at best despite having the accolade of being the creator of what is probably D&D's most popular and expansive setting.
