Eh, Windows-only software certainly seems like a valid reason to me. People are also allowed to have personal preferences.
Don't get me wrong. Linux is the only desktop OS I use. I've daily driven it on my personal machine for about 5 years and at work for about 6 months. I vastly prefer it over the alternatives, but I do put up with a fair amount of annoying bullshit (mainly graphics, sleep, and Bluetooth issues) that would probably be less of an issue on Windows (or macOS). I still use Linux because I can tolerate those problems in exchange for the benefits, but I can see why other people wouldn't want to.
All of these things have already been disclosed.
ActivityPub is a public standard. https://en.m.wikipedia.org/wiki/ActivityPub
kbin is open source. https://github.com/ernestwisniewski/kbin
Lemmy is also open source. https://github.com/LemmyNet/lemmy
Google is your friend.