It's more complicated than that: Hubzilla was forked from Friendica in 2012; Friendica adopted ActivityPub in 2018. So they have different implementations. I re-checked, and not listing Hubzilla by default has valid reasons. The main one is that, unlike other platforms, hubzilla servers do not declare their communities, so our crawler can't see them. Also federation is opt-in, so many of the communities in that directory are not reachable. We will include Hubzilla communities, but only after specific suggestions through the form we have on the site.