I think it might be the other way around.

No, its pretty clear from reading his entire response that he understands exactly what sargable means. "just use indexes properly" is pretty ambiguous, but the rest of his comment provides context. It doesn't make much sense to me to be indexing on fn(x) for predicates unless an alternative sargable predicate is too complex or not possible, but that doesn't mean that it's never the best solution.