They were a race of psykers before the fall as well, seing as it were their seers who saved the race by predicting the fall and warning enough people about it.It's also worth noting that the Eldar only have to be super careful because there's a chaos god waiting specifically for them, who has a unique bond with them. Pre-Slaanesh, they were a race of latent psykers that didn't live in constant fear of daemonic possession.