The questions raised by Minos were fair. Indeed, it made little sense to say that someone who had died had entered the Upper Realm, and also, for someone to know about something that should be extremely secretive and known only to that person.
But Minos and everyone around Mylecent and Ysolt were just ordinary people who had never had contact with Celestials.
Minos was the least ignorant of all, but the individual he had already come into contact with and talked to was a True Celestial and not just a Celestial. He didn't know many things that these two super-experienced women knew.