It provides a runtime which developers can link their games against, but there is nothing which enforces or mandates this. You can ship a binary that makes 0 us

It provides a runtime which developers can link their games against, but there is nothing which enforces or mandates this. You can ship a binary that makes 0 use of the steam runtime, and this binary could be blatantly broken on various distros (though Valve does have some testing in place to make sure games actually launch, but this testing might be limited to Windows)