Why does it seem like they're arguing that? I think that it's supposed to really be: cause -> (plaques = cognitive disease), and the fraud was (cause = plaques) -> cognitive disease.
As I took it, understanding that there was a fraud doesn't mean that continually clearing the plaques wouldn't have a good chance of holding off cognitive disease indefinitely.
I'm simply arguing that the plaques could be the proximate cause and the problem addressed by this treatment could be the remote cause. The OP is being vague in the way they stated it, but it feels like they're saying the plaques are irrelevant. That doesn't seem to fit what we know: the evidence that cognitive symptoms are downstream of plaques is pretty compelling.
I also don't think "fraud" should be used in this discussion at all.
With all that said, if this One Weird Trick can clear/prevent the buildup of plaques (and thus the cognitive symptoms downstream of them), that's just a best possible outcome.
As I took it, understanding that there was a fraud doesn't mean that continually clearing the plaques wouldn't have a good chance of holding off cognitive disease indefinitely.