That’s not the same as requiring the followers collection to always be associated with an actor subject (the inverse of what you referenced in the spec). What you referenced is discussing the actor of the Follow activity and what side-effects should happen when an Accept(Follow) is received. AFAICT, no one is suggesting that non-actors can perform activities.
I suppose you might claim this is a redefinition of an AP redefinition of AS2 back to the original AS2 Follow activity definition, but I’d need to think more about that. 