It's because they:

1. Forked an existing app that has all the features you listed.

2. Admit they don't really know the language and tech it uses.

3. Said upstream was too slow, but I don't see any(?) PRs from them on upstream besides the 2 that they list in the post. Their fork appears to have an extra 3,000 commits.

I'm not super against them doing this, but it's pretty easy to see why people don't like it. Hell, this is the same group that upgraded one of my side-projects from a few years ago and improved it into their 'baibot' matrix bot, so I wish them all the best. I like people making money from OSS, more power to them.