As the thread you linked to explains, Discourse Hub can push notifications for any Discourse site, even ones not hosted by the Discourse team, but it only pushes live (instant) notifications for sites hosted by the Discourse team. If you add a non-hosted site to Discourse Hub, you’ll get notifications, but at a slower schedule.
Nonetheless, I don’t think that’s what the question is asking about, but I may have misunderstood:
Discourse Hub can already send iOS push notifications to from all of the Discourse instances within it. Non-hosted sites send notifications more slowly, but they do send notifications.