Have you ever actually submitted a credit card dispute? It's like a little miniature arbitration. You can submit whatever documents and justification you want.
My point would be that the argument would be clear. It stands to reason that a business cannot consider an unlawfully collected payment to be legitimate. If you can demonstrate that you attempted to give them notice of cancellation and they ignored it unlawfully, then that's your justification that the payment wasn't authorized. You can, of course, revoke authorization to collect payment anytime you want.
This is a really common kind of dispute that businesses have with credit card users. Typically, their response is that the user did, in fact, authorize it because they agreed to a variety of terms that mandated this and that for cancellation, and so on and so forth. In essence, if they can get the user to admit that they gave the original authorization, their argument would be that it was never withdrawn because the user previously agreed to some T&C buried definition of how cancellation would be defined.
The counter to that, of course, now, is new, which is to say that those terms are unlawful and thus negated. It seems completely sound to me as a strategy.