If this is true, then I see no other blame than Airbnb themselves. If you make a platform allowing reviews in such a way that the "seller" can edit the reviews "users" make, then that's never going to become anything other than scammy. Even allowing a review to be scrubbed/removed by the seller is bad. There should be a mechanism to work with the platform to handle fake/false reviews, but the seller should never be able to do that on their own.