It looks like they have their own "vibe" and rigorously enforce it in their own community. That's fine, it's their service after all. Feels a bit strange to turn away potential new users when people are looking for viable alternatives
I don't think Tildes is turning anyone away, not sure where that narrative comes from. Anyone can request an invite from the owner themselves.They are just not looking for explosive growth like some of the other services are.
In fact I think the invite system is an effective way to keep the boys away as the free-for-all signups are easier to game.