Given their tortured struggle with double negatives, imagine the verbal discharge if they
taught articles.

Most English teachers (ESL included), are neither grammarians nor linguists. The Doc gave an explanation as to what constitutes a double-negative using her linguistic knowledge and training.

For the rest of us the explanation is much simpler:-

A double negative is usually produced by combining the negative form of a verb (e.g., cannot, did not, have not) with a negative pronoun (e.g., nothing, nobody), a negative adverb (e.g., never, hardly) or a negative conjunction (e.g., neither/nor).

Whilst this many not be STRICTLY true from the viewpoint of a linguist, the fact remains that it is CONSIDERED to be true by teachers of English.

Therefore, using the more 'common' definition, 'She's not unattractive' is considered to be an example of a double negative.