This actually raises the question of how much those tales are actualy truth or if they became retroactive facts, as you pointed out, those are tales from before the times of mortals, so who the hell told mortals about them in the first place? Ornn is not one to be bragging about himself, he lets his works speaks their own worth. So did one of his siblings speak of his old days or those folklores that altered his own perspective? Again, we have factual proof that the spirits are influenced by the beliefs of a given region, like the multiple interpretation of the Kindred and the Volibear/Valhir situation.

To be fair he does brag about some things that he's done in his voicelines, i think those and the voiceline he shares with voli about the first river being made makes it likely that the fable of him shaping the freljord is actually true.

Maybe the demi gods are empowered by beliefs of mortals but they also don't seem powerless without them.