I was looking up "nektogon" in the dictionary compiled on the Dothraki Wiki, and "Nektogon" meant "to cut", in the sense of sculpting, not as in cutting into something to eat. I'm not sure if it carries the meaning of "to maim", which is closer to the meaning I get from the context.
I believe the Duolingo course is written/supervised by the author himself, while the wiki, though excellent, is kind of a reverse-engineering of the language by fans. (The copyright of the language belongs to HBO, not to David J Peterson, so he can't directly provide grammars or dictionaries unless HBO allows it; which means he can't directly write a wiki – though he's been very helpful in providing specific answers and examples, which fans use to compile the wiki).
Long story short, if you find a nuance in Duolingo that's missing from the wiki, Duolingo generally takes precedence (unless it's a typo or mistake, which happens; it's in beta, after all, and not totally reviewed yet). Well spotted, btw.
The course was supervised by the author himself, but two of the course contributors were authors of the wiki! The problem is, David hasn't had time to comb through every sentence and vet it (and honestly, Duolingo does not make it easy for him to do so).
So while in theory evidence from sentences in this course should outweigh the definitions I wrote in the dictionary, in practice it's not there yet.
... also I should confess that both the entry on the wiki and this sentence were, um, composed by the same person, so if there's any contradiction, you can blame, um, him ;)