My prior point was to illustrate the essential lack of enforcement of any rules, hence by the definition of anarchy as a "society without coercion".

From the lens you are using, that of governance, The Culture definitely matches "stateless society based on voluntary free association" — this is, as shown though historical real-world usage, compatible with "direct democracy", where votes have no intermediaries.

This voting structure was demonstrated in the books: there was a direct-democratic decision about "should we have a war?", most said yes, those who voted "no" were free to disassociate, and not merely in a purely theoretical fashion as the technology at their disposal enabled complete independence to a degree utterly impossible for any human on Earth. In the author's own words, "the Culture kind of fades out at the edges", rather than having a discrete boundary in the way we are used to on Earth today.