To whom does "Lo Seitz'u" refer?
Ramban #1: It refers to Aharon, who was permitted to perform the Avodah 1 in his capacity as Kohen Gadol, and to his sons in the future, but not to the present (since they were On'nim, and not permitted to serve on that day).
Moshav Zekenim (from Sanhedrin 18a, Sifra Mechilta d'Milu'im 28): It teaches that if a Kohen Gadol's relative died, he does not follow the bier (closely. When the procession moves to a new area), he appears (enters the area from which it left). 4
Since Aharon and his sons were forbidden to leave Pesach Ohel Mo'ed, how was Elazar able to burn Parah Adumah on Nisan 2 (Rashi, Gitin 60b? He needed to go outside the Machaneh - Bamidbar 19:3!)
Moshav Zekenim: A Mitzvah is different (this was permitted).