Earlier quoted context omitted.
That's meaningfully different. "Can't be copyrighted" doesn't mean "can't be sold", or "someone else owns the copyright". It just means someone can copy and resell the generated portions without payment/licensing.
That sounds like a fucking nightmare if you're running a marketplace for what is essentially intellectual property.
I assume that, if it is true that Valve isn't allowing games with generated images, it's because (they feel) the legal status could change, not because of the current status.