Certainly sounded like a hot mike. You can hear the usual humming sound of two transmitters interfering. Also, there's some background noise suggesting it's a hot mike.
The hot mike could have been in 5512, the aircraft sitting on the runway waiting for its takeoff clearance: after the event they said "
5512, we never heard *any* transmission...". Well, when there's a hot mike, there's usually only exactly one aircraft which did not hearing anything...
![smiley :-)](https://forums.liveatc.net/Smileys/default/smiley.gif)
Everyone else would at least have heard some humming and garbled comms...