It's true that Mueller alone has not the right to subpoena the president.
A subpoena isn't exactly written with the power of the executive branch though. It's a request to the judiciary branch of which the judiciary branch can say 'this, make it happen.' And Trump is subject to the judicial branch's power. If a judge tells the executive branch they want someone in front of their court to answer questions the executive branch can complain however much they want but that's going to happen.
A subpoena isn't exactly written with the power of the executive branch though. It's a request to the judiciary branch of which the judiciary branch can say 'this, make it happen.' And Trump is subject to the judicial branch's power. If a judge tells the executive branch they want someone in front of their court to answer questions the executive branch can complain however much they want but that's going to happen.