Hmm, if anything that link seems to imply it's invalid - plus it's Quora, not exactly a high quality reference.

The rule is "an" is used in front of a word that starts with a vowel sound[1]. "An hour" is correct because the "h" is silent. All modern (native) English dialects pronounce the "h" in history, ergo "an history" is incorrect grammar.

[1] https://en.wikipedia.org/wiki/English_articles#Distinction_b...