The convoluted responses of Fox and PhD are proof of your statement.
Fortunately, FTs in China are not responsible for teaching grammar.
Given their tortured struggle with double negatives, imagine the verbal discharge if they taught articles.
The horror.

I've never mentioned double-negatives myself beyond saying they are too much for me, so I certainly don't even bother with a slight effort in their direction, let alone a struggle.

I suppose Turnoi can seem convoluted sometimes but that's because he's a linguist, and linguists tend to be a bit fanatical about getting grammar and stuff right. So that's not a nasty criticism of him at all.

I like to intervene sometimes in basic Mandarin; the oral side of the language, and that's only to be helpful. Because if you say 'mei ge ren bu...you simply will not be understood by a Chinese; whereas you may be understood by a foreigner with a good knowledge of Mandarin; because he/she will make more allowances. Alas that won't help you much.

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.