CFAA could (and likely would) apply for remote vulnerabilities i.e. exploiting SQLi on someone else's servers; but in the case of local privilege escalation like this particular case all the exploiting/testing happens on systems owned and controlled by the researcher, so it doesn't violate CFAA and doesn't need any permission from Valve - the breach happened with authorization from the system owner.

You need permission to pentest someone else's systems, you don't need permission to pentest software on your own systems even if that software is written by someone else. In an enterprise setting it's possible that you have signed a contract where you agree not to do such testing or not to publicize its results; but violating that would be a civil matter regarding the terms of that contract, not a felony in respect to CFAA.