St. Athanasius wasn't excommunicated by the Pope.

That's debated.  It appears that he did sign a docuмent, under duress, but them rescinded it.  If it was done under duress, it would have been illegitimate of course.

But Sean's point is still legitimate.  Not only did they not stop hearing Confessions, but the Catholic bishops went around installing non-Arian bishops in sees that had been usurped by Arians.  "Jurisdiction" wasn't a particularly well-elaborated concept in those days anyway.  It was only later when "Auxuliary" or in the East "Chor" bishops started to become consecrated to help the primary Bishop with his duties that the distinction between a Bishop with jurisdiction and a Chor bishop without jurisdiction came to be clarified.