This hanachah must be performed specifically by a kohen, as that is the mitzvah that the pesukim refer to when they indicate that the mitzvah must be performed by a kohen. However, if a non-kohen were to light the menorah in the Kodesh, that would be the hanachah. Since the hanachah must be performed by a kohen, a non-kohen would not be allowed to light the menorah inside the Kodesh. Hence the Rambam had to describe a scenario whereby the menorah was removed outside of the Kodesh so that a non-kohen could light it.
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