I would agree - no microphone needed. Biden's words were all recorded for prosperity by the media and probably the campaign using conventional methods.
But I still think he was getting information, either audibly or visually, from an external source. I don't see him moving quickly on his feet. At least not quickly enough to respond to any line of questions. I suppose he could have memorized (mostly) his answers for the pre-determined line of questions. His only response to Trump was his trademark giggle. Except at the end when he had that "deer in the headlights" look.
Well from his perspective its no more difficult than for any on air news personality who has a producer talking in their ear. How much surgery or other invasiveness had to be performed we will never know.