Not only is it impressive considering the TIA's rendering limitations, it's completely smooth despite having no frame buffer and waiting on the 6507 to feed it data during the blank portion of the analog display signal - but the fact that the audio is crusty, but perfectly understandable using the two-channel 1-bit audio generator on the TIA.
I have to admit I skipped through the video until the audio because I am not technically competent enough to understand a lot of it, but it sounds like it was using the audio capabilities of the 2600. The audio quality was by no means good, but you could hear the dialogue in the video clip well enough.
Some dude did something similar for Gameboy where he was running GTA v on it. I'm gonna guess this is doing something similar, where it's leveraging the higher storage capacity of modern storage to basically stream video.
