It doesn't need to say the word linking explicitly as it is implied, because when you link to libraries you essentially make them a part of your program by including a portion or the whole of that program and the GPL or compatible licenses have to cover your entire program. I mentioned that binary blob drivers aren't linked to show that they are not a part of the kernel in any way, shape or form. They are discrete pieces of software that can interact with one another in a way similar to plug-ins as modules can be loaded or unloaded while the kernel is still running.
The most important line of the license in this matter is as follows: "Activities other than copying, distribution and modification are not covered by this License; they are outside its scope." Stated plainly: if a binary blob doesn't copy or modify the GPL covered kernel code then it is not bound by the GPL itself, period. As to distribution, that would be down to Valve or any other distro having to comply with the requirements to make the source available for any GPL covered works they provide.
This is a very clear cut case, the only time it seems ambiguous is when you have FUD-spewing FOSS zealots involved.