It can definitely be mastered at 48-bit, but it will just be put on the CD at the standard Redbook (IEC 60908) HD-CD 16-bit/44.1 kHz PCM. HDCD is a technology that Microsoft has rights to that allows for 20 bits and full backward compatibility with standard CD-DA format. I highly doubt that this is a HDCD though, as they aren't too popular. (I've only seen one in my life.)
The studios use higher bit rates and sampling rates to increase sound quality (or at least that's the plan). The reason for over sampling (which I assume they probably do to 96 or 192 kHz) is to prevent aliasing near the top end of the CD's bandwidth (~22.05 kHz by Nyquist Theorem). This is generally combined with some kind of dithering and noise shaping. That is also done on the decoding end. The bit rate in mastering would have a much less marked effect than over sampling, and some would argue that the only thing noticeable would be that they spent more time and care mastering that particular selection. It's what is known as a "confounding (or lurking) variable" in statistics since changing one (the bit rate or sampling rate during mastering) will usually change the other (the care spent in mastering the CD).
