Sorry for nitpicking, but you seem to remember incorrectly. Actually, む used to indicate "n", so む could be read "mu" and "n".ん used to indicate 'mu' before 'n' (if I remember correctly)... so there are two ways to read わからん 'wakaran' and 'wakaramu' = "I don't understand"...
八百万神 is usually read "yaoyorozu no kami". 八百万 is indeed written やほよろづ in historical kana orthography, but the pronunciation is "yaoyorozu", similar to the case that む is pronounced as "n". If you want use "yaho" for 八百 to represent historical kana orthography, 万 also should be "yorodu".the 8 million gods... yahoyorozukami
Sorry for nitpicking, but you seem to remember incorrectly. Actually, む used to indicate "n", so む could be read "mu" and "n".
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:
The hentaigana 无 from which む was derived was used to derive ん and standardised in 1900...
Yes, ん indeed used to represent "mu" or some other sounds, but these are more likely considered exceptions, as the wikipedia page mentions that these usages are found in 土佐日記 hand-written by 藤原為家, but generally, む was used for "n" sound.The truth is apparently more complex than that, as alluded to in the text immediately preceding that which you cited:
You seem to have several mistakes here.
The む character was derived (is believed to have been derived) from 武, not from 无.
无 is not a hentaigana. It is a kanji. Hentaigana are not kanji. Rather, they are equivalent to hiragana, although now mostly obsolete and no longer in common use.