Paul Farquhar
Legend
D&D Beyond divides all content into "sourcebooks" and "adventures" and Stryxhaven sits very firmly under "sourcebooks". WotC only ever called it a sourcebook or setting book in their own marketing. You are probably thinking of early reviews which went "OMG this is an adventure!".Not always. It was initially announced as a sourcebook, but the marketing got increasingly adventure-y as time went by, and by the time the thing was released, the adventure component comprised well over half the page count. I suspect (without any evidence whatsoever) that the content and focus of the actual product morphed similarly from setting to adventure in development.
Actually, it's just a framework on which an adventure can be hung. Most setting books link encounters geographically (i.e. with a map). Stryxhaven links encounters temporally, using an academic year structure. But it doesn't include an actual story. It's up to the DM and players to provide that.