The website is called Fedidevs, but it used to be built against only Mastodon. Not the Mastodon client API, but Mastodon's very specific inner workings.

The issue wasn't just that starter packs only worked on Mastodon. It was also that starter packs could only contain Mastodon accounts, and only Mastodon users were even able to join the site because the technical process of joining explicitly looked for features that are only present on Mastodon and nowhere else.

It was blatantly obvious that the maintainers of this site firmly believed that the Fediverse is only Mastodon when they built it, and that they refused to accept the truth when someone pointed out that there's more to the Fediverse than Mastodon later on.

Eventually, after quite a while of operation, they added specific code for certain Fediverse server applications. They added specific code so that Misskey accounts could be added to starter packs. They added specific code so that Pleroma accounts could be added to starter packs. And so on.

The whole thing is not built against any API, much less against ActivityPub. It's built against the inner workings of specific Fediverse server applications. Changing that would require a complete rewrite of the entire backend.

So in order for Hubzilla channels to be added to starter packs, the FediDevs maintainers would have to add specific code to FediDevs so that it supports Hubzilla channels.