tar.md5 = remove .md5 and you got a tar archive
However, that won't help you, because it contains mostly binaries
I think you're missing the point of how open source works. The OS itself may be open source. Anyone can distribute it, and they can even charge for it. If they modify an open source program - they have to release the source. However. They can write additional programs, such as TouchWiz, or hardware drivers, make them closed source, and distribute them bundled with open source OS. As long as the program is not a derivative of a GPL program, they are free to make it closed source.
TL;DR Base OS is open source. Builds that are customized for specific hardware don't have to be, and seldom are