Oliver Cowdery initially wrote “the word of Nephi” in 𝓟; then he inserted inline the plural s at the end of word. The level of ink flow is unchanged, so probably the correction was virtually immediate and in accord with the reading of 𝓞, no longer extant here. Typically, the text prefers “the words of X” over “the word of X”, where X refers to a person other than deity. For instance, there are four instances of “the words of Nephi” elsewhere in the text but none of “the word of Nephi”. For another example of this plural usage, see the discussion under Alma 25:9 regarding the phrase “the word(s) of Abinadi”.
Summary: Accept in Helaman 9:15 Oliver Cowdery’s corrected reading in 𝓟, namely, the plural words; the virtual immediacy of the correction in 𝓟 as well as usage elsewhere in the text supports the plural words in the phrase “the words of Nephi”.