Does SteamOS offer something significant over other GNU+Linux distributions?

It's pre-installed on the device and pre-configured. If you can configure and troubleshoot a GNU+Linux system I don't think it has anything unique to offer to you. The thing where it boots directly into Steam big picture mode should be implementable on other distros as well. But most people can't or don't want to learn how to set up a Linux distro so it's actually a big deal for the average gamer. It's also made by a big corporation that people already trust, which might be something Gamers Nexus feels more comfortable implicitly endorsing by including it as a platform for their benchmarks.