[OT] The "genetically superior intellect", indeed!

I was thinking about something in wrath of Khan that has always bugged
me. When khan is monitoring kirk and spocks conversation between the
enterprise and the genesis cave, and spock is giving kirk repair time
estimates and says "If we go by the book captain, hours would seem like
days". Spock's inflection practically tells that they are talking in
code. He explains it to Kirk over the open channel, for christ's sake!
Kirk and Spock are always going on about what a genius khan is, and he
was supposed to be a big general/ ruler back on Earth. Couldn't he
figure out that Kirk and Spock would have some kind of protected
communication mode or code? Kind of obvious and short sighted for such a
"genius", don't you think?