"Canonically," the Forgottem Realms is connected to many worlds via portals, including Earth. Lorwyn can be its own distinct setting that is not part of the Realms, and also have a portal connecting it to the Realms. Also, as of 4e, the Feywild invaded the isle of Gwynneth and established the nation of Sarifal, which was also previously home to a hidden wood elf enclave like Rivendell. Thematically its on point.
I don't know the details of this supplement, so I can't say how much impact it will have on "greater realms lore" without reading it. But given that its one of these mini "DLC supplements" and not part of the main book, I figured its just a fun little crossover. D&D is a "multi-verse" basically.
All somewhat pointless however. This is not the first time Wizards (or even TSR for that matter) has added unpopular additions to the Realms. Your best bet is to just pick what you like. As you've seen, everyone has their own opinion on what "fits" or doens't. Maztica, Kara-Tur and Zakhara were not part of Ed's FR, but they are "official" products. The Spellplague was controversial to say the least, and I personally find the lazy full retcon that is the second-sundering nearly as offensive. Hell, the Moonshae Isles were not even Ed's, they were lifted entirely from Douglas Nile's books and just pasted over what was already there. There's even an image out there of Nile's Moonshae's map hastily taped over the original.
