NEVER let your man split the trap. Bad traps lead to nothing good. They had that guy locked up and then stupidly went for a steal, opening the door to the split. That step through and shot took strength.
Edit: The guy never gave up his dribble, so I can see why the D moved. However, the defender on the inside should have closed towards the offensive player (and towards half court) when the player made his move towards the sideline. Instead, he drifted down towards his own basket and opened a lane. Sucks for them. Still a lucky shot.