Indo-European Poetry and Myth

Reading "Indo-European Poetry and Myth" by M.L. West, on pg 132-3 he discusses that there may have been an indo-european idea that when gods disguised themselves as men, their eyes were unblinking and their feet glided, not touched, over the ground. He cites Mahabharata 3.54.23 & Heliodorus's Aethiopica 3.13.2 as specifically saying this. But, he notes, instead of being derived from IE, they could be late transmision, even, he specifically proposes, Alexandrian. But countering that he supposes they probably are derived from an IE source, because Homer in Iliad 3.396 has Helen recognize Venus/Aphrodite disguised as an old woman, because of her sparkling eyes, and in 13.71 Ajax recognized Poseidon because of his leg movements.

But the problem that I keep coming to in a lot of these issues of derivations from IE is, how was it passed along? If Heliodorus wrote about the god waving shining eyes and their feet not touching the ground when they are disguised as humans, from where did he learn it? Was it passed down from early IE speakers in 6,000 BC to Heliodorus in 400/600 AD without any other of the millions of people that must've heard it writing it down? Its one thing to say that the old bards and orators learned to write about gods taking human form, but seems like another to say that they learned about the shining eyes and gliding feet, without writing it down somewhere.