The Atari 2600 had no video memory. The video signal out was generated on the fly. This was not just pong, but for every game that ran on this system. I think it generated an interrupt when a scanline needed to be started for output to the TV and then it was up to the CPU to provide the data on the fly.
A lot of vector games worked this way too. For vector monitors this makes total sense because the main program has control of the electron gun and doesn't even have the constraints of timing, except how many inches of line it can draw between refresh cycles.