For now, you can draw the line for what he means at GNU since SteamOS is that while ChromeOS is not. But if someone made a GNU/Linux that holds your hand everywhere like ChromeOS, it would probably be "not real Linux." So maybe the better designation is "Linux for nerds."
SteamOS has the benefit of being basically entirely open and modifiable. Heck, it has KDE Plasma for its desktop mode.