Someone on the thread linked to the previous version APK from a site called uptodown (glanced at it, didn't seem to be malware, but didn't do any real forensic analysis). I enabled sideloading on my phone, tried to "update" the app, but the OS then refused to install it, claiming it was invalid.
Can't tell if it's an OS level safeguard or an app-level one. Very frustrating, either way. I had my version of Nova launcher locked for years on both my phone and tablet after the acquisition, but when I got a new phone I obviously had to install the latest-greatest, and at that point I forgot to disable the auto-update flag...