Here in 𝓟, Oliver Cowdery initially wrote retreat back. He immediately crossed out the adverb back and supralinearly inserted into (there is no change in the level of ink flow); then he continued inline with the following text (“the wilderness again”). 𝓞 is not extant for this part of the sentence, but there is no room for this extra back in the lacuna except by supralinear insertion. The source for the intrusive back, of course, is its occurrence in the following yea-clause (“yea even back by the same way which they had come”). This correction does not mean that “retreat back” is impossible; this expression is found elsewhere in the text:
And, to be sure, there are other examples of the verb retreat without back (six more of them), including this nearby one in Alma 58:18: “I caused that my men … should retreat into the wilderness”.
Summary: Maintain in Alma 58:24 the corrected reading without the adverb back: “they began to retreat into the wilderness again”.