Right, the GPU dump on crypto crash is one side of the bad scenarios for Nvidia (the other side being extremely high crypto demand which drives gamers to consoles and they loose a large chunk of their GPU gaming business).
NVIDIA makes the guts for Switch and AMD does Xbox and PS so gamers moving to consoles maybe isn't that bad. Gamers moving to mobile platforms might be more of a problem although I'm not sure if anyone would really say "dang this 1080 is too expensive so I'm gonna play Candy Crush instead"
But Nvidia does not control whether crypto crashes or booms. They have to have a strategy keep their base (GPU gaming/Datacenter) market intact in either of these scenarios. I don't think having their own coin is a viable strategy for these scenarios, it will just drag them deeper into the crypto world.
However they're exposed to those crashes and booms already. So I think the point Phil was making is that NVIDIA/AMD would actually have more control if they developed their own algo or otherwise embraced crypto in a more "official" way.