This is only the source code for third-party free software projects used in the Kindle, including Linux, busybox, libpng, and freetype. It doesn't include any of the Kindle application code. It's not new - they've been publishing these since the first Kindle release in November 2007, as required by the GPL and other licenses. See for example:
http://www.mobileread.com/forums/showthread.php?t=16247