The singular workman, the reading of the printer’s manuscript, is an obvious error, and the 1830 typesetter was clearly justified in emending the text to the plural workmen. Both the singular and plural are identical in pronunciation, so it is quite possible the original manuscript also (incorrectly) read workman here. The context definitely suggests that king Noah would have had many workmen, not just one.
Occasionally in the history of the text there have been mix-ups in the number for the compound morpheme -man. There is, for instance, one case in the original manuscript where Oliver Cowdery wrote down workmanship as workmenship:
In this case the error in 𝓞 may be due to phonetic identity, just as it may be in Mosiah 11:10 for the spelling workman. Elsewhere in the text, Oliver consistently wrote workmanship (4 times in extant portions of 𝓞, 11 times in 𝓟). There is one other case of the plural workmen in the text, and in this case Oliver got the spelling correct in 𝓟:
There are two cases where the number was mixed up for other words taking the compound morpheme -man. In both these cases, the error occurred in the printed editions:
See under these two passages for discussion of freemen and watchmen. There are two other words that take the compound morpheme -man—namely, king-men and spokesman. For these two words, the compound morpheme does not show any variation, at least with respect to the spelling of the vowel. It should also be pointed out that there has been the occasional mix-up in the number for the word man. For discussion and examples of manuscript difficulties between man and men, see under 1 Nephi 15:35.
Summary: Maintain in Mosiah 11:10 the 1830 typesetter’s emendation of workman to workmen; the error in 𝓟 may reflect an earlier error in 𝓞 where the scribe misinterpreted the number for workmen as Joseph Smith dictated the text.