It seems you're focusing a bit too much on the specific example. It's about: "Hey, this is a place and role this PHB core species can be found in and has a relevant contribution to the adventure locations of that setting that you can use to find something to base your character of one of those PHB core species."
If there aren't enough Dragonborn on the planet for every major city to have a Dragonborn [something], but there are human [same something] in almost every major city, your setting seems insufficient to be that 5.24 setting.
And mind you, it's perfectly fine if Forgotten Realms isn't that "optimized for 5.25" setting. It doesn't need to be bend to be so unless you want to. But I think a setting that is designed with the PHB core races and classes first and foremost in mind would be something interesting to have.