Isaac's article on "releasing the Zalgo" is for API creators/designers, not API consumers - the consumer of Understream shouldn't have to worry that his function might sometimes block the event loop and sometimes not, it's our responsibility to ensure that we're consistent.

The pattern above for detecting if a function calls it's callback synchronously or not is actually from his article on releasing the Zalgo, and is recommended behavior for exactly this situation, where the user passes in some function that might or might not be asynchronous.