Is the wiki page (http://wiki.dothraki.org/High_Valyrian_Verb_Moods#Special_expressions) wrong in this regard? Or does the mistake lie in this course? The course appears to be teaching the reverse of what's on there. I.e. Duolingo says vocative + infinitive = let X do Y, but the wiki says vocative + infitive = may X do Y, and also dative + infinitive = let X do Y.
Note the lesson text:
Finally, commands can be issued to non-second person referents. Such commands are often translated with "let" in English, even if they're not explicitly requests. For example, when your boss says "Let me see what you've been working on", they're not really asking permission. In High Valyrian, while there is a distinction between requesting permission and non-second person commands, the English translations may be unhelpful in distinguishing between the two.
Well, "Let the princess seep" is somewhat ambiguous in English: it could mean either "allow the princess to sleep" or "may the princess sleep." My understanding is that the dative = allow, and the vocative = may, but that perhaps sometimes (as with English) the distinction can be blurred. Again, we would have to ask dedalvs himself to be certain.