Articles and quantifiers are different parts of speech in most grammars. Articles are specifically different in having no independent meaning, more similar to prepositions than to adjectives (which some is classified as in its use as a quantifier that we were discussing here).
English just doesn't have an indefinite plural article. This is not actually that uncommon - German has the same property, for example.
Quantifier words and articles are all the same part of speech. It's still true that the plural of a(n) is some.
There are other ways to use some - for example, it can be singular too - but that doesn't affect the question.