I don't think that's as meaningful (nor accurate) as you think, as (supposed) decentralization isn't the same thing as non-existence (or inexact existence, to paraphrase your OP). After all, the same could have been said about the Front de libération du Québec (FLQ), which was organized in a cellular structure, with each "cell" operating in a (supposedly) autonomous manner, without a "central command". And yet, the FLQ was as real as the dead politician's body they left in the trunk of a car for the police to find.

There are several "Antifa" organizations that are "chapters" of the "Torch Network" (https://torchantifa.org/chapters/), though not all of them have "Antifa" in their name. Decentralized, yes, but with a centralized website you can visit. As for there being no "charter"/"rules"/etc., the Torch Network has a "Points of Unity" document on its website (https://torchantifa.org/points-of-unity/), and I'm pretty sure that if any of its "chapters" ran afoul of said points, they would be disaffiliated pretty quickly.

In any case, I see the usual rhetoric around Antifa being just "an idea" as being so much "Nothin' to see here. Move along." Anonymity is one of the core strategies of Antifa--on the streets it takes the form of the black bloc approach (i.e. everyone wearing black clothing to conceal their unique/identifying features). Likewise, in public discourse, the "just an idea" charade is merely a tactic to condition Joe Six Pack and Sally Soccer Mom to think of Antifa as a figment of the conservative imagination.