Lemmy only for now. You're not the first person to ask that. Tbh I have no idea how similar piefed or mbin/kbin are to the Lemmy API. I'm not ruling out expanding support, but I would likely prioritize supporting all platforms (e.g. Android, Linux, Windows) for Lemmy before expanding support to other APIs.
With the lemmy donation push recently and the problematic views of the main devs being talked about again I am sure you will hear more people asking about piefed and *bin.
Cool. I'll give it a try with the android release.