I really don't think he's wrong, at all. I think what you're experiencing is the exception.
I know a few groups in the UK, and there isn't one of them that doesn't have 3PP adventures in the mix, and often 3PP settings too. Anything else 3E, like classes and subclasses and races (not connected to 3PP setting books)? That's a lot less likely.
Kickstarter has huge reach, and leads normies who aren't deeply involved with RPGs, who maybe could scarcely name an RPG other than D&D into seeing and buying 3PP stuff (which often looks cooler than or as cool as WotC's stuff - in fact I'd say WotC have upped their aesthetic game in part because of that). Plus, it's on the shelves at every FLGS, and when you search online for D&D stuff you're likely to come across it. On top of that, WotC themselves are massively endorsing 3PP stuff by putting on D&D Beyond, which means it'll reach even more people.
I'm not saying WotC shouldn't make adventure or w/e, but the idea that it's unusual for people to have 3PP stuff for 5E seems incredibly far-fetched to me. Hell, sometimes 3PP stuff is the route in to D&D, even - via stuff like Humblewood.