In what sense are coroutines dual to (sub)routines?
I think this a misanalysis of the "co" prefix in this case.

"Coroutine" is "co" in the sense of "co-worker"; something that works together with another.

The term precedes by a long way the gross overuse for programming concepts of the prefix "co" in the Category Theoretic sense of a dual of another concept.

(Yes, there is editorial content there. ;-) )