The thing about the Steam runtime is it's literally just a different way of packaging it as far as the dev cares, and overhead for containers is much smaller than a full VM, so performance impact is minimal. Containers were originally created to make cloud deployments easier to automate because all the most important dependencies are packaged and there's a stable interface to the OS regardless of host, and it replaced heavier virtual machines for most autoscaling web apps. Doesn't need full virtualization and or guest kernel, etc. Easy to suspend for hibernation too, which is great for portable gaming too.