True, in actual settings.
However here we are dealing with imaginary, even narrative, settings, where things are simplified for the sake of storytelling. And example of such a simplification is "if my aliens don't produce vowel sounds then none of their names should have vowels" -- irrealistic, yet self-consistent).
Sorry if I made it sound like it was a real-life rule -- and if I made it sound like hard rules. I mean neither.