That’s correct. If the priest were reciting the parts of the priest, and most specifically the consecration, he would be concelebrating, not assisting in choir. Being in choir is precisely a way of being present at a liturgy as a cleric (thus maintaining the distinction from the laity) without exercising one’s function/office. Bishops can also celebrate in choir if they choose to have a priest celebrate the Mass (bishops don’t act as concelebrants for priests).
One distinction, if a bishop is present, the bishop must officiate the Mass, which means that the bishop gives the opening and final blessing.
Thanks for the qualification. I had forgotten that detail.
If we are getting into distinctions, is this correct if the Bishop is outside his diocese?
I believe so. At a recent wedding (I’m a wedding sacristan) the bishop of Fargo presided in choir within the Diocese of Ft. Wayne-South Bend. I gave him a mic so he could do the parts required of him (not that a mic is absolutely necessary, but we need to wire celebrants when there are video crews).