It is a common practice to have www as a SAN, even if it is just for a redirect. It costs nothing extra either as it is often included by default in my experience.
They use lets encrypt so its all free. It just seems a weird thing to complain about for a site just starting out. Sure it could be more polished, but also people could just link to the correct url.
Not weird at all. If you are listening on www. then you should have a certificate for it. Super basic stuff.
They're listening on all subdomains. Its not like they explicitly enabled www. https://foo.justapedia.org/ has the same problem. IDK, i just think this is all incredibly nitpicky and really doesn't matter
No, but it is their fault for not caring that people will use www and no-www interchangeably, and not having a SAN and redirect for the secondary one. That's objectively a usability problem for the website.