Open source development is only a full time paid job for a small number of people, for everyone else it is largely driven by passion. In principal, I'm not sure anyone should tell anyone else what they are allowed to do with their free time.
But if you had taken time to read the first sentence on the link, you would see that there is in fact a key difference:
> Kallithea [...] is [...] source code management system that supports two leading version control systems, Mercurial and Git
You’re not wrong per se but this community is a place to showcase interesting technology and this is one of them.
Maybe the author just wanted to build something to practice code or to do something different than GitHub.
Innovation (and evolution) happen via mutation - it’s wrong to ask people to conform to set standards in a forum dedicated to interesting bits of knowledge.
But if you had taken time to read the first sentence on the link, you would see that there is in fact a key difference:
> Kallithea [...] is [...] source code management system that supports two leading version control systems, Mercurial and Git
This is something that alternatives do not do [1] [2] [3] [4]. There are likely further differences if you read the documentation at https://kallithea.readthedocs.io/en/latest/readme.html
[1] https://github.com/go-gitea/gitea/issues/458 [2] https://github.com/gogs/gogs/issues/125 [3] https://gitlab.com/gitlab-com/blog-posts/-/issues/261 [4] https://www.gerritcodereview.com/roadmap.html