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.