I believe I read somewhere that since coming to the PS3 generation they started moving more of their systems to C# and C++ in order to be able to share more code with the other Sony WWS.
Rather, the guy who moved it from Common Lisp to Racket informed the audience it is actually used like an advanced templating system to allow the system/core devs to interface with sound engineers and programmer-designers (not artists, but the guys coding the art into Maya3D templates and what not) to generate proper header and class structure for their C++ engine. These guys are not the core C++ devs, so they needed something to streamline their work for many test builds and that could be used to assist programmers that were not trained or designated for core systems programming (with C++ and all that entails). This actually irritated one guy in the audience, and made me laugh because CL variants have many more native code compilers, and the native C transpiler in Racket (although useful) is touted as something you would use (it is for systems porting), because their JIT and native code compilation receives a lot more work.
Anyway, this is all here-say. I would say watch the video and read the slides. Although Racketeers will be disappointed that this dev from ND, sometimes apologetically, admits they used MzScheme and had not really moving ahead with updating to later Racket versions (hence the name change) or many other features.