The Constitution governs. Not the DOJ opinions.
You are correct in when you say you do not know.
As legal theory and practice one can make a knee-jerk argument that one cannot be judge in their own case, and similarly not in the case of immediate family and so on and so forth.
In practice, presidents have and do pardon immediate family members, best buddies, people who have made contributions to the president in order to get pardons, etc and no one has been able to stop it because the power of the president is considered to be unlimited. All of those pardons are contrary to "the fundamental rule of justice" but there they are.
Bill Clinton pardoned his brother and go him out of prison. And he pardoned Marc Rich who was a fugitive and on the run but had been one of his major campaign contributors. Boom! About as biased as you can get and being a judge in his own interests. The Constitution does not necessarily tidy everything up in all matters, nor does that mean the DOJ and others can project on to it what the framers should or should not have done to fill in the blanks. An in-house opinion by the DOJ is just one more in-house opinion.
The analogies over to the judicial system are of limited value. The president is not acting as a judge when he grants pardons. He is exercising executive power that the constitution has given him.