Argument over whether cuм Ex is infallible or not is a waste of time.....
You're veering off topic. cuм ex was essentially only used as a prime example by +Fessler of a pope's teaching that is not infallible, but never the less is taught that it is infallible by those who should know better. Hence the title of this thread. I referenced Fr. Fenton's teaching to demonstrate exactly this - he is one who should have known better.