I love their theme as angels of order and such. Almost alien in nature compared to the mortal races, and objectively heavenly.

But what does that have to do with dinosaurs?

A part of me thinks that this is just a leftover theme of the WHFB, where some guy made a lizardman army because it was cool, then over time the army's lore was adjusted and retconned to be what it is today, but I don't want to believe this. I'm hoping that there is some sort of mythology that found dinosaur bones and believed they were good gods or something, and Warhammer used that for the Slann.

Otherwise, Holy Dinosaurs just seems like random themes pieced together.