Scribe 2 of 𝓟 initially wrote “taking upon themselves”, then virtually immediately corrected the themselves to them by crossing out selves. As discussed under Mosiah 25:23, the expression “take upon them(selves)” occurs in the Book of Mormon text with both them and themselves. In each case, we follow the earliest textual sources. Here in Alma 13:8, the critical text will accept the corrected reading in 𝓟, “taking upon them the high priesthood of the holy order”.
Summary: Accept in Alma 13:8 the corrected them in place of the themselves that scribe 2 initially wrote in 𝓟.