This is an area which the GIRM itself does not provide sufficient detail, and we must rely on the prevailing practice that has been handed down.
It is in general understood that the central focus of the Church is the reserved Sacrament in the tabernacle, to which we express our reverence by the gesture of genuflection. However, this focus is moved to the altar during the celebration of the Mass, especially within the sanctuary. Hence in the GIRM paragraph you quoted it instructs that genuflection to the tabernacle within the sanctuary is omitted during Mass, but only before and after the ministers enter the sanctuary. An exception to this is when the priest or a duly appointed minister approach the tabernacle to remove the reserved Blessed Sacrament for Communion and when afterwards replacing the Blessed Sacrament to the tabernacle; at which times the minister will genuflect.
It is my understanding that the “otherwise” in the subsequent paragraph refer to when it is outside of mass or when the tabernacle is not in the sanctuary.
I have never seen a lector genuflecting to the tabernacle as he approaches the ambo in Rome or any of the cathedral celebrations or any papal mass before. I have not seen it even with some of the most conservative congregations I have been to. So, either nobody has gotten it right or it is not supposed to be understood that way.