Because that society is aware that it will be attacked by authoritarian factions and will be armed for self-defense, what is the problem of understanding this?
We have fanatic pacifist societys in the game right now, you know? and they are viable in this unrealistic science fiction game, (And yeah absolutely impossibles to survive in time based on human history and the game logic!)
There are plenty of countries that have no militaries IRL.
There are no countries that have no states.
It would be impossible to model without either making anarchy unrealistically powerful or unplayably weak. Unrealistic not in the sense of reality but in the sense of anyone putting this OP thing in their game, though.
The two most obvious paths for modelling anarchy are to either make it playable, which means it's a few adjectives in infoboxes at worst; at best it's a free ship and army spawning associative collective that can create enough stuff to fight any defensive wars without resorting to console cheats; or it's a bunch of semi-rebellious sector/planetary governors who are under zero control of the central bureaucracy/player. At that point you could just say the whole galaxy is a single state and have no serious differences.
Anarchy mostly doesn't work in Stellaris because the player character is the civil service. If there's no state there's no player agency. You can get the same riveting gameplay as an anarchy by typing "observe" in the console and watching the bots duke it out.