Yeah, programmers tend to use "isomorphic" rather loosely as a fancy way of saying that two things have roughly equivalent semantics, but Joe here is using the term with a specific technical meaning--two objects in a category are isomorphic if there is an invertible morphism between them. So Optional<T> and Either<T, Void> are isomorphic because we can define a Swift function f that transforms an Optional into an Either, and another function g which transforms an Either into an Optional, and these functions f and g are inverses of each other, in the sense that their compositions are the identity on Optional and the identity on Either, depending on the order in which you compose them. Another example is that (Int, (String, Float)) and (Int, String, Float) are isomorphic types, but they're certainly not the same type.
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