Although not the same, both are affected by the ruin... ruin is simply a consequence because of lack of resources. Casshern simply has the best of both worlds... a body that doesn't rust like robots because it's capable of regenerating itself, and a body that doesn't require nutrition like that of humans. So no matter how you look at it, the ruin doesn't affect Casshern, but it does affect humans just the same... just not the same way as it does robots.

But as you say, the human downfall was already in effect before this whole ruin business came forth.

By the way, in case anyone missed it... the chick robot in this episode was called "Sophita". They clearly didn't mention her name enough during the episode, so you guys might have missed it.