It’s easy to leave the gear down and it’s been done more than once, especially if the PM gets distracted during the ‘gear up’ call. However an A320 would be very unhappy indeed if you tried to land with the gear up. Was the belly landing intentional?
And I’m struggling to understand going around from a belly landing as it seems the inevitable damage to the engines left them with a double flameout.
Unless somehow (and I don’t know how - they forgot AND the gear config failed AND they didn’t do the checklist) they landed without the gear and then were shocked into going around at the resulting cruncher.