It might not be a thing for “normie” users, but the tech-overlapping crowd has been all about domain name based handles for quite a bit, and many are registering domains specifically because of it. This isn’t something they’re doing on a whim, they’re doing it in direct response to demand.
Two @ symbols? I think you're thinking of how Mastodon works. Bluesky doesn't work that way. Your username is the domain itself, not @username@domain. If you own the domain brian.pizza, your username can be @brian.pizza. If many users will share a domain, they are simply given a subdomain -- @brian.someserver.lol, etc.
