The english is not quite right. The correct translation would be "This knife is not sharp enough".

Saying "This knife won't cut well" infers extra information, ie you know it is not sharp enough for whatever substance you are trying to cut.

If I were to translate 這把刀不夠鋒利 into English, I would surely translate it as "this knife is not sharp enough" as you suggested, as it conveys the meaning fully while sounding natural at the same time.

However, as you can tell by comparing the ID's or dates of creation of the English and the Chinese sentences, in this case it is the English sentence being translated and not the Chinese. My translation, 這把刀不夠鋒利, may be a bit too liberal, but I believe it is far more likely to be said by a native Chinese speaker to express the idea of "this knife won't cut well" than a literal (and perhaps "accurate") translation like 這把刀切得不好.

The Chinese and English languages differ so substantially that translators sometimes have no choice but to sacrifice a bit of the original wording to make the output more natural-sounding. I believe the sacrifice is worth it here.

That makes more sense. Going English to Chinese it sounds ok, its just going chinese to english it doesn't (: