I knew them exclusively online, a few I've met in person a few times, others we've only ever interacted online for the better part of 20 years.
As the world gets smaller, I think this kind of thing will become more common. I think the borders were smaller 20-30 years ago online, and now there is a bit of nationalism/tribalism separating folks, in part because so much of online interaction is now a "service" and not just a newsgroup, special or general interest forum, or IRC.
I think if some of the decentralized systems become more usable/popular we might start to see these adhoc communities form and disappear more organically online, but probably not.
I've tried to go back, to EQ or other MMOs, and they're much harder to get a group of friends from now. You used to see the same people in your server, sometimes meeting them multiple times as you leveled through the game. They became known to you and even friends. The changes to make the game more open to solo players and with more instancing you just don't run into the same people more than once anymore.
The COVID lockdown definitely made me think more about the sense of community and belonging we developed with those games/guilds. Somehow in-person interaction seemed harder when you are a teenager. The limited sensory experience of typing and using mouse inputs versus even speaking seems to have made it less threatening or inhibiting. Maybe that’s why I don’t actually feel as trapped as many people have reported feeling.