Safari is a lot more strict about cookies than Chromium or Firefox, it will straight up drop or ignore (or, occasionally, truncate) cookies that the other two will happily accept.
I had hoped when writing this article that Google would look at Safari and see that it was always strict about feel comfortable about changing to be the same. But doing so now would unfortunately break too many things for too many users.
Thanks for pointing that out -- I've updated the article and given you credit down at the bottom. Let me know if you'd prefer something other than "kibwen."
An unspecified "implementation detail" is essentially another way of saying that it doesn't work.
I've ported my account on ActivityPub a couple time, and it's a horrendous experience -- not only do I lose all my posts and have to manually move a ton of bits, but the server you port from continues to believe you have an account and doesn't like to show you direct links on there anymore.
The latter could probably be easily solved, the former needs to be built into the spec or it will continue to be broken.
I had hoped when writing this article that Google would look at Safari and see that it was always strict about feel comfortable about changing to be the same. But doing so now would unfortunately break too many things for too many users.