No, I still don't think I understand what he meant. if "unproven" and "disproven" aren't synonyms, then my guess would've been that "unproven" means something that has neither been proven or disproven. But that wouldn't make sense in the context of "can't be unproven" or "unproven had become proven".

Are you saying that unproven means something that was thought to be proven, then more info came out that made the proof look less convincing? That makes some sense, but then his statement that "unproven had become proven" still doesn't totally work with that definition to me.