It was demonstrated by engaging people in the mythology, just like at Expedition Everest. Themed entertainment is not a text book, it is not all spelled out. There are plenty of fictional animals, but there is one common strain through all of those chosen by the creative team that dreamed up the park. No aliens, no purely fictional creatures, no animals just made up for the park. All of the animals chosen are rooted in cultural understandings of the world: folklore, myth, legend; not contemporary story telling. Years of research and work go into a park. It was no accident that aliens were never part of the plan until chosen by financial executives.
Rhoda's career is dominated by an obsession with research into places and topics, so much so though Disney decided that the best way to tease Pandora at the D23 Expo was to pretend that he had taken Imagineers on another research trip. Science fiction is just not the sort of thing Rohde has done in his career as a creative leader. You can't actually visit Pandora or do more than just talk to James Cameron, and as cools that may be it is not the same as trekking across the Himalayas to put research a fictional creature (to understand its cultural origins).