The Lemmy implementation is compatible with Mastodon. Thats because it works in a very similar way by using Announce, with the main difference that the actor has type Group and not Person. In Lemmy posts are also owned by the user and announced by the group, which works just fine with Mastodon.
I dont know which problems Hubzilla had with Mastodon compatibility. Maybe their forum implementation works in a different way from Lemmy, or there were changes on the Mastodon side before Lemmy started federating.