It's still early days for SteamOS, which was released in its first beta form last year and has, until now, demanded a newer UEFI machine to work.
But Valve is gradually building and improving the OS, with the latest version now compatible with older BIOS systems. Plus, it makes it easier to install as a dual-boot configuration, which you can do in "Expert mode".
But the news also comes with a warning: "There has been very little testing on this, especially any kind of dual-boot setup. So don't install it on any machine you are not prepared to lose."