that post will have been a text post, not a link (those are likely broken now, and certainly were broken a year ago due to a bug in the misskey 12 codebase inherited by firefish and forks. modern versions of misskey just fixed that a couple months ago)
the username thing does not completely break federation, but it will randomly confuse instances. there's a 50/50 chance whether an instance will get the correct user it asks or not, and once an instance resolves a user once it'll have a similar 50/50 chance for each profile update (icon change, sidebar change, etc.). of course, if there's no conflicting user for a community (or vice versa) then federation will be fine.
the specs are so open ended that i doubt real interoperability will ever happen. you can break interoperability with basically every other current software out there and still be compliant with the specs