I think that what he was referring to was the probably much oversimplified, but frequently-encountered explanation that the hiragana character ん was once a "hentaigana" of the character む, or, put another way, was used in writing to represent the 'mu' spoken sound, before it was re-purposed to exclusively represent the 'n' sound. The truth is apparently more complex than that, as alluded to in the text immediately preceding that which you cited:
「ん」の字形で「む・う・も・ん」の音を示す単語に共通して用いられている