Both the ground and tower conversations are already up on LiveATC.
Before pushing back, these two flights stepped on each other's transmissions to ground. That's a clue. While still on ground frequency, ATC directly advised both flights of the similar numbers on the frequency. Both flights acknowledged.
Delta somehow started rolling even thought the takeoff clearance was for 31C and he was on 4R. I Wonder if the Delta crew every heard of a pilot named van Zanten?