grammar - How is "plenty" a pronoun in "plenty of time"?

The Oxford Dictionaries list "plenty" as a pronoun. Example sentences include:

I would have plenty of time to get home before my parents arrived

There are shops in plenty

But pronoun by definition is a word that substitutes for a noun or noun phrase. How is "plenty" a pronoun in these examples? What noun/noun phrase does it substitute for? In contrast, Merriam Webster lists it as noun, not pronoun. I think in the sentence You will have plenty to draw from "plenty" is a pronoun, but it being a pronoun doesn't make sense in the two sentences above. How is the usage of "plenty" in those sentences different from that of these ones:

I need a large amount of money.

There was food and drink in abundance.

Both "amount" and "abundance" are listed as noun in dictionaries.