That makes sense if the application is, like, a SAAS app, and the scope is, like, "don't employ credential stuffing or test any of our 3rd party dependencies that have not given us permission to be included in this scope".

But this is software people install on their desktops, and Valve has no say in how security researchers approach that stuff. Valve can and maybe even should exclude LPEs from their bounty scope (if that's not what they're focusing on right now), but they can't reasonably ban people for publishing vulnerabilities they've scoped out of the only mechanism they've provided for submitting and tracking vulnerabilities.