I'm curious about this as well. The phrase "non... più" translates as "no longer" or "not... anymore" and in this sense it's not a negative comparative as in your translation. It seems in Italian (and I'm guessing the other romance languages) that distinction can't be made without context.
What I'm trying to point out is that these two phrases in English, which have different meaning, both translate to the same Italian phrase...
This is no longer entertaining. / This is not more entertaining. = Questo non è più divertente.
I thought the same and then researched it a bit (DL has me questioning my English Grammar now - and once I was an A student!).Both Cambridge dictionary and http://alt-usage-english.org/anymore.html define any more as quantifiable, e.g. "I don't want any more pie." and anymore as meaning "nowadays" or "longer". Explained in this sentence "I don't by books anymore because I don't need any more books." Maybe I will remember it now!
"do you want any more fish?" it is separate. "I don't like DuoLingo anymore because I lost my last heart on this question" it is a compound word. Confusing? I don't know the grammatical rule (native speaker), but I think it is separate when referring to a quantity of things, together when referencing time.
Divertente is an adjective, my dictionary gives comparison with Piacevole=amusing, entertaining, enjoyable, or Buffo =funny, amusing . So both sentiments should be acceptable. Duo strangely sometimes wants more literal translations and sometimes not. Piacevole and Buffo are two different emotions as several above have noted. It would seem divertente can be used for both and we would rely on the context to know the difference. One of the snags of isolated phrases. I try not to get hung up on it