As a Texan...that person's assertion makes no sense. I have never heard anyone else ever claim that y'all should be used as a singular pronoun.

Do you mean you wouldn't only use y'all for singular? I assume you would say "how y'all doing" to one person or multiple people.

I've spent a lot of time in the US South (Georgia and Tennessee) and cannot say I heard people every using "y'all" in the singular.