Now that Explorer's Guide to Wildemount is out, I'm curious to know how much of Matt Mercer's homebrew content is now considered "official". Since this is a first-party WotC official D&D sourcebook, does this mean that his homebrew Exandria setting and its associated lore is now to be considered an official setting, on par with the Forgotten Realms, Eberron, Dragonlance, etc.?
What about a setting makes it official, and does Matt Mercer's setting of Exandria qualify for that?
Obviously his rulings and other house rules (basically anything he's said during his shows that deviates from RAW) are in no way official, no more so than when Jeremy Crawford tweets about how he'd rule something, so I don't so much care about that.
I'm more interested in his homebrew content, specifically his setting. (I'm also interested in how much of his homebrew magic items, spells, etc., are also considered official – I'm guessing for things like that it's simply "if it's in that book, then it's official" – but the focus of this question is on the overall setting and its associated lore.)
Why am I asking this?
This answer to another question of mine somewhat tentatively includes a reference to material from this new book, and since I asked for official lore in that question, I'm somewhat confused about whether or not it really counts as "official lore" (not to scrutinise that answer–I think it's good to include in that answer anyway; see also my comment under that question that expresses some confusion over this issue).