Long-time Sync user here, now on kbin. I'm hopeful that cross-compatibility will be considered, like Hariette's Artemis app which aims to support both kbin and Lemmy (and both Android and iOS). I'd love to jump back onto Sync, but probably not if I have to move off kbin to do it.
Honestly? I would pay the premium for every quality app for lemmy/kbin. You had Sync, Relay, Boost, RiF on Android for reddit. If those were 'ported' for activitypub I would pay for each and every one. Even if it meant I used only one of them.
This is not the real Sync dev, it's someone pretending to be him. The real account for LJ Dawson is @ljdawson. He confirmed that via his Reddit account. The community he established for the Sync for Lemmy app is @syncforlemmy.