Yes, but they don't have to modify the kernel source. Chances are that what they need is drivers for PS3-specific hardware. If implemented as loadable kernel modules, GPLing the code can be avoided. It is the practice used by many embedded Linux vendors.
It is not that simple actually. There is a lot of controversy whether a kernel loadable module is a part of the kernel or not, because it "interacts with kernel in a very intimate manner" [Linus Torvalds]. I haven't been following this debate for a couple of years now, so I can't tell what's happening today.