You say it's a non-representative performance for Biden but that's not really true. Biden has not been doing many interviews. He does prescripted events where he can use a teleprompter.
You have to go back to Regan to get fewer press conferences. He even skipped the traditional superbowl interview that is a notorious softball.
This all suggests that his team knows he can't handle an interview and are avoiding it. If they thought he could handle it with his polling they would have him doing them all the time.
After the debate performance he needs to do something to show he can string some thoughts together in a coherent way. I think this Fridays interview is a desperate last shot to do that.