www is dropped from the url, so I don't believe that's the case

It is, there’s a long special list for domains that host arbitrary subdomains, GitHub pages is one of them. It’s explained well in https://youtu.be/0-wB1VY3Nrc

Wait, seriously? Their default behaviour is so broken that they have to whitelist a ton of sites, and screw anybody who slips through the net (or is trying to start a new service)?

Most browsers use the Mozilla public suffix list [1] that is frequently updated, and a new service can easily submit a new entry. This list is used for many different features.

This list is necessary anyway, because you have top level domains that look like .co.uk, so you can't just split the domain name by dots and take the last component to determine the top level domain.

So, not saying I unconditionally like the fact they are hiding important information in the URL bar, but I would not say their default behavior is that broken.

[1] https://publicsuffix.org - probably the list being mentioned in the video linked by the parent commenter, I guess

This list seems to be actually used to determine how cookies can be shared across domains, but not by Safari? Does this mean that Safari might have different behaviour as to when cookies can be shared across domains?

Chrome is/has/was special-cased dropping the display of the "www." prefix, and that one only, for a while now.