Nothing is a “new interpretation”, it’s an existing pattern that interactions sometimes go to attributedTo.inbox. For example when you Like something, if it doesn’t have an object.inbox you check the object.attributedTo.inbox and send it there. What this FEP does is generalize the behavior of “crawl up attributedTo until you find an inbox”.
It’s unreasonable because actors don’t have to be users. Actors can be anything. The spec explicitly notes this. A network of actors where actors are only users is not workable because various implementers are already doing things that don’t represent users… they’re just lying and calling them users, because they have to lie in order to be compatible with the incorrect interpretations and incorrect assumptions.
Some people are against multi-typing, so there’s pushback there as well.