What if a Yisrael does the עבודה of a לוי? (Erchin 11) II

The Minchas Chinuch is based on the Rambam who doesn't write that a Yisrael who does the avoda of a Levi is chayav.

However, the Rambam and Minchas Chinuch seems to be against the Gemara. The Gemara says אילימא זר ממש, it is already written, and Rashi comments that here would be a חיוב מיתה by the עבודת הלויים as well for a זר ממש.

The Brisker Rav answers that the Rambam is against the Gemara but the Rambam paskened like the Sifri Zuta and the chiyuv misa for a Yisrael doing avodas haleviim was only a הוראת שעה.