This is just legal cover until such time as its possible to enforce no child exploitation imagery, no copyright stuff, etc. It doesn't matter if they are able to enforce it, Valve can use this policy as cover if they ever get sued. Don't overthink the motivation. They will not even have a bulletproof way to detect AI imagery as it evolves every single day as an arm's race and detection is a full-time job. Even a FAAN…
Are people actually trying to detect AI-generated content? That would not only be pointless and futile; the threat of false positives would be enormously detrimental to anyone creating legitimate work.
It is such a ridiculously bad idea I'm dumbfounded that anyone smart would be trying to do it.