Plural indefinite pronouns?

Sounds like your commenter is from an earlier tradition of analysis. These days, amongst other determiners, bare quantifiers used in a context where a substantive is expected, such as the subject or object of a clause, are generally considered pronouns.

Certainly Wikipedia disagrees with the assertion that indefinite pronouns cannot be plural:

Indefinite pronouns, the largest group of pronouns, refer to one or more unspecified persons or things. One group in English includes compounds of some-, any-, every- and no- with -thing, -one and -body, for example: Anyone can do that.

Another group, including many, more, both, and most, can appear alone or followed by of.

Even numbers, which are normally thought of as quantifiers, can occur stand-alone in substantive contexts. I don’t know what you want to call those, but they’re definitely more than a mere adjective.

What difference does it make whether you call them pronouns or not?
It doesn't change their properties or their usage or their meaning or their syntax.

Go right ahead and call them pronouns if you like -- or call them quantifiers or determiners.
It makes no difference either way. It's just a name, not a description.
Definitions are for mathematicians; they are not facts, just presuppositions.
And, in the case of POS definitions, they are quite often incorrect presuppositions.