I would think that the rules are the rules. Sessions even used the word "required", when talking about himself and why he is no longer involved.
And I would think that Rosenstein (and Mueller) are required to be recused and it's not a matter if it being their choice, as Sessions implies.
And I would think that it's their supervisor Sessions, who is responsible for making that happen.
I really dont understand your point. It seems you think it is an automatic kick in with no decisions that have to be made
Please go read recusal procedure