It's sort of by WotC.
It's published by WotC via DMsGuild (like for example any of the much shorter products they've put out to benefit the Extra Life charity, or like the Wayfinder's Guide). But it doesn't look like anyone on the WotC RPG team worked on it. Instead, its lead designer was James Ohlen, who was one of the folks responsible for the original Baldur's Gate I & II games. He released a DMs Guild setting guide / adventure based on those games through DMs Guild a while back ("Heroes of Baldur's Gate")—then got hired by WotC in their video games division. I wonder whether perhaps this new book, seemingly a follow-up to "Heroes," was already under independent development by Ohlen and then the decision was made to release it "by" WotC since Ohlen was now a WotC employee.
Still, this is interesting—to my knowledge it's the first ever new book-length product (albeit half the length of a regular WotC 5e book) that WotC themselves have released on DMsGuild with a print-on-demand option. I wonder if a door is opening...