Yeah, but in the film Kirk says to the security guys that "you're K'normian merchants now". To me, that implied that they were supposed to be pretending to be from that race.
Or maybe it's a nation rather than a race. One thing I hate about Trek and most other SF is the way they treat the two as interchangeable, as if every political entity were defined on purely racial grounds. There are Americans whose ancestors come from every country in the world; why can't there be people who are K'normian by nationality but human by birth, or Vulcan by nationality yet Denobulan by birth, or what-have-you? (It would certainly explain why there are two completely different-looking species called Ktarians, and several different ones called Rigelians.)
In any case, we have seen instances where characters impersonated members of another species by using audio communication only. Perhaps Kirk's plan was that they'd use the K'normian scout ship and identify themselves as K'normians over the radio if challenged, but wouldn't send a visual transmission.