How is the ancom argument of real communism or socialism having never been tried functionally any different from the ancap of real capitalism having never been tried? Both seem like no true Scotsmen.
One could quibble and say anarchist communism has existed, but examples are primitive societies or blips in history. Examples of pure capitalism are comparatively rarer, medieval Iceland and Ireland are go-to examples cited by ancaps.
To an extent, while I'm a strong descriptivist, if we said every country claiming to be something is that thing, we'd have to accept the Third Reich and China as socialist.