Early KNC indicate the host would be a embedded sodimm board so not a RPi.  Then again it doesn't really matter.  The host is relatively low end stuff.  It needs a tiny amount of memory, a low end processor, ethernet, and some sort of serial connectivity to the ASIC boards.  An embedded linux board will work fine, so will a RPi.  I assume they simply have more experience with this board so they are using it.