[dvt]

Iota is still defined in terms of S and K. More technically, Iota is an "improper" combinator (see pg. 779 in [1]). Therefore, you can't axiomize Iota, so it would be hard to argue that it's more "fundamental" than S and K.

[1] https://books.google.com/books?id=1xEVkzuX5e0C&pg=PA779&lpg=...