The author states outright that this is not unique to Chinese, it's just much more prevalent than in American English.

不错 is literally "not bad", but it's more positive than the American English equivalent, being basically semantically equal to 很好 (lit. "very good", although in practice just plain old good/OK). You can even say seemingly absurd things like 很不错 "very not bad" (= excellent); or you can tamp it down with 还不错 "also not bad".

Funnily enough, in British English, "not bad" is high praise; but you still wouldn't say "very not bad".