Containers are great for development and production on your own infrastructure, or shared infrastructure like GCE or AWS. Security can be had from doing inspected builds, self signing, etc.
For consumers, however, it's a completely different ballgame.
By the time serious hacking is an issue through Steam, I'd expect containers will be that much better anyhow. For all they can be criticized, and regardless of whether you think some other approach would have been better, they're getting the "trial by fire" treatment. By hook or by crook, in another year or two I expect they'll be as secure as you could ask for.
Worn most of the time while a typical user is awake, goggle hacking will represent a very large target with widely varying rewards.
I think kordless is claiming that using Docker here could increase the severity of an attack; otherwise it doesn't seem like putting up another barrier could hurt security, even if it is later broken.
Everything falls to hacking eventually. That's the nature of it, at least till now.
I would note that Docker is primarily a tool for developers and operations folk who are also the author of the software being run. Docker itself is not the risk here, but using it for some use cases may very well be.
But it would still help in preventing such bugs as https://github.com/valvesoftware/steam-for-linux/issues/3671
HN discussion: https://news.ycombinator.com/item?id=8896186