They mean that they need to fiddle with those specific video registers twice a frame, which is 100 or 120 times. Vblank timing isn't too hard, the base system has an interrupt for it; scanline timing is hard though, you either need to carefully time your loop (and redo it for 50hz systems), or poll on things, or use extra hardware built into the cartridge (mappers).

I don't think there was an IRQ for sound, but it might have made sense to poll the audio unit rather than the graphics unit for some reason.