I thought I gave you proof: Ubuntu comes pre-loaded with desktop GUIs and other user packages, such as office, web browser, etc. - All of which are not needed when building an OS from the base up.
Is Debian the best choice? Maybe? Maybe not? That is for time to decide.
I really don't want to participate in a "Which Linux distro is better?" thread, as this question doesn't apply here.
The real question is "Which distro is the best base?" If steam was to put real time and effort into it, compile their own kernel and code, I'd say LFS, however, it lacks a package/update management system. I can understand their decision to choose an established distro with package management system. (e.g. apt/synaptic/aptitude) Any number of distros can fill this bill, Arch, Gentoo, Cent, Debian, and so on, (provided they support the hardware on which they are being placed, and have no excessive/conflicting software.)