> Don’t you already need to know the people before you are part of the Stammtisch?

You do. And the classical eponymous Stammtisch isn't really inviting to newcomers. I've spent time in German Wirtshäuser (but maybe it's more extreme in Bavaria) sitting at a table next to the Stammtisch alone but they would never invite you to their table.

It's just not how it works, at least in Bavaria. Usually the Stammtisch is an extremely tightly-knit group of locals that frown upon everything new. Somehow over many years you might earn your right to an invitation.

Also if you go to a Wirtshaus it is easy to spot the Stammtisch (the actual table - it always has a bell) and even if it is empty you may not just sit there without permission just like in some kind of religious ritual.

Of course everyone is free to use the term to their liking and you call a regular meeting in different locations a "Stammtisch" but that is not how it is traditionally.

I’m getting from the article that you can get invited to these as well as keeping existing friendship going.

Alone != lonely.