Even if your reasoning is strictly speaking right, the issue is very different in peertube and in youtube (or at least could be, i don't think they have a good

Even if your reasoning is strictly speaking right, the issue is very different in peertube and in youtube (or at least could be, i don't think they have a good recommendation system yet): peertube is a federation and not a monopole. Distributed systems allow to have a clean separation between the an exploration algorithm, its actual policy and the instance hosts. The policy can be very much user-specific, easily with a federated-instance granularity and could be run on an instance which has no link with the content it's searching.