That just makes them Fanatic Spiritualists with Biological Ascension.The degree of the Elders' psionics is clearly psionic ascension level, sorry. They're straight up body-hopping and linking all their slaves psionically with it. They also in general have a very spiritualist atmosphere to them, especially in the first firaxis game.
This is especially obvious with the Volunteer from Enemy Unknown, who both has the psionic power to take a colossal spaceship away from earth and also according to the directors use their powers to escape the explosion that follows.
Being a few Hundred years in Advance does not make them Materialists. Age can make technological lead more then Ethos and Spiritualist can make them older/more cohesive just as well. Just ask the Spiritualist Fallen Empire
I actually made a Steam Guide on how to make "Imitation Empires" in Stellaris. I have begun to include the thus far revealed 1.5 mechanics into it:
http://steamcommunity.com/sharedfiles/filedetails/?id=689073779
It might be worth a look.