I was at microsoft from 2008-2011 and definitely saw some C++ in the kernel, even one driver using operator new for its allocations which I thought was a little nuts at the time. I also personally introduced some user mode .c files, it was just my preference. I don't think there's much rhyme or reason to which is used, although MS does lean very heavily towards C++ over C, seems like more and more as time goes on.

As for this directory tree... I found it creepy, like maybe source was leaked. The only inaccuracies I found relative to my memory of parts that I worked on was (1) I don't think they have done well tracking files that are part of static libraries and (2) what I presume are new files introduced by folks who came after me.