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I'm pretty sure they don't mean "Playback of this pre-rendered MP4 was powered by a single GeForce RTX 2080 Ti!"

"a real-time technology demonstration using Unreal Engine 4.22 ray tracing"

Directly from the description of the video at Unreal's yt channel.

Source: https://www.youtube.com/watch?v=Qjt_MqEOcGM

I don't know, there doesn't seem to be any mention either way, if it's not pre-rendered it is slightly better but still, I don't understand why a packed room at GDC of all places would be wowed by this.

I think that it uses Unreal Engine entails that its rendered in real-time.