> And if retrieveURL is nonblocking, then iteration is nonblocking.

Something I mention in the article/paper/whatever is using operations that are technically blocking to call functions that happen asynchronously. So there are certainly uses for sync iteration tools (like _.each())

I think it depends on where you put the async assumptions. Do you know/assume that the function itself takes care of the async aspects, or do you know/assume that the caller must take care to set up that structure?

If the answer is the former, then use whatever structure you want, because the async is taken care of for you. If the answer is the latter, perhaps you need a iteration function that works asynchronously.