I think that poster is saying that Swift's generics are not as good as C++'s, and even more not as good as Scala's. "Miles away" is ambiguous, but I think it's

I think that poster is saying that Swift's generics are not as good as C++'s, and even more not as good as Scala's. "Miles away" is ambiguous, but I think it's "miles behind" in this context, given its proximity to "even" and "let alone."