Michael seems to be on close terms with the iOS audio development team. I suspect his confidence is not unfounded/out-of-the-blue. It might be worth being in touch with him to get a feel for that.
Despite being deprecated, Apple has continued working on it – i.e. fixing IAA bugs in iOS 13 leftover from iOS 12. IAA could be one of those technologies that they deprecated and then realized that they haven’t come up with a suitable replacement.
Don’t get me wrong, Audulus working as AUv3 would be lovely, but it seems like there would be cases where running it standalone would enable it to run things that it can’t as AUv3.