Having recently started doing lightweight programming on my c64 this was very questionable to me, especially considering the Amiga (the much more powerful followon to c64 couldn't even get close to this level of performance).
Following the mastadon link revealed the secret:
"@electron_greg This is incredibly impressive! What kind of wizardry is this? There's no way this is running on a stock C64. I assume it's using an accelerator of some kind but I don't see anything sticking out of the cartridge port..."
"Correct. It is equipped with a "RAD" cartridge which is effectively a co-pro in the form of a RaspberryPi Zero."
So the heavy lifting is being done by the Raspberry Pi's ARM CPU, not the c64, which is I'm guessing is essentially being used as a fancy frame buffer to display the Raspberry Pi's output.
This is still REALLY impressive though to be able to interface the two this way, and I'm glad to see this. Well done electron_greg!