It's also that symbols and gestures evolve in a social context, people, language, customs and culture changes. It's not unthinkable for symbols to get a worse connotation over time because a group used them a lot. So the question then becomes, do actual white supremacists consistently use the symbol as a way to identify themselves? Eh, I don't think that's the case anymore or in meaningful quantities. But after the populairty of the hoax white supremacists did co-opt it for a while. Is it still completely meaningless if a racist mass-murderer flashes the symbol?
So yeah, indeed it started as a hoax, but the implication that the ADL only included it because they were somehow gulled by a hoax is incorrect.
(As an aside, the OK-gesture is quite culturally different and dependent on context in the first place. Had a mate who while drunk almost started a fight with some Turkish dudes by flashing it, since apparently it mimics the asshole and basically means "you're an asshole". And in France it apparently means that you're a zero, since it looks like a zero.)
[0]: https://www.adl.org/education/references/hate-symbols/okay-h...
4chan once decided to promote the idea that the "a-ok sign" (making a circle with your thumb and finger) is a white supremacist coded symbol, basically as a test of how easily gulled the press is about such issues.
Well, the press got gulled, and today displaying the symbol can get you fired, or banned from certain locations. The ADL considers it a hate symbol.