What the heck is "not", anyway?

In the examples of the OP, enough is a determiner for the ellipted noun people:

Enough [people] are present to form a quorum.

The determiner is negated by the adverb not:

Not enough [people] are present to form a quorum.

The understanding of what an adverb is, and what it can modify, has broadened over time:

An adverb is a word that modifies a verb, adjective, other adverb, determiner, noun phrase, clause, or sentence...

Adverbs are traditionally regarded as one of the parts of speech. However, modern linguists note that it has come to be used as a kind of "catch-all" category, used to classify words with various different types of syntactic behavior, not necessarily having much in common except that they do not fit into any of the other available categories (noun, adjective, preposition, etc.)

wikipedia.org