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.