I've been working for the past week on a new kbin app. At the moment, it's just a prototype and only the "Feed" view has content (you can't sign into an account or post anything yet); you can see threads, thread images, thread comments, thread links, and thread votes/boosts. You can also change the "Theme Mode" and "Instance Host" in the settings.
Here's a link to the source code (contributions welcome for those that know flutter): github.com/jwr1/interstellar.
For testers, here's the link to the latest working build: github.com/jwr1/interstellar/releases (AppImage and APK).
Feel free to post any bugs or suggestions in the comments, on GitHub, or on the Interstellar magazine.
Again, this is just a prototype. If someone else is working on a better kbin app, I might just use that instead.
For what its worth, ernest said he wanted to come back working on his mobile client starting next year again. No idea how far that project went before and how much work it would be to get that in a usable state though.
Ernest will no doubt correct me if I'm wrong, but I think he's also said he'd prefer to leave aps to those for whome it is a core strength.
I think he knows it's something the community would like, so I think the post you link above was in reaction to Artimis unfortunately going offline and the App (for now) dieing with it (since nobody seems able to contact the developer). I suspect it's something Ernest will progress 'in his free time' if no other apps which live up to his ambitions are developed first!
I think he said even further back that he wanted to eventually come back to it but I also certainly did not wanted to discourage OP from working on his own one, especially an open source one that could get picked up by someone else if needed. I think the more apps we got, the better. And hopefully some of the Lemmy exclusive apps will support kbin too after the full API release.
My priority is definitely the development of kbin-core, especially now that work on the app has started. For now, I prefer to focus my attention on adapting the API to the latest features. It may turn out to be necessary sooner than expected ;)