“Exclusively?” No. But obviously its initial appeal was to the more tech-savvy and FOSS-centric sort, and it’s byzantine enough to jump in that it dissuades many newcomers who try.
But ActivityPub does seem to look like it will pull in larger services (like Threads) so in the end “protocols over platforms” may win out by default, sorts like WebKit/Blink/Chromium has. Not everyone gonna use Brave or Opera, but the mass of Chrome users will still feed back in some fashion.