It is certainly true that Coolreader is released under the GPL2:
http://sourceforge.net/p/crengine/cr...i/master/tree/
And as per the standard GPL terms, if you make your personal modifications of a GPL project available publicly, you must, well, not contribute back your changes, but certainly make them publicly available for anyone, including the original project, to download and/or use, if they so desire. As a byproduct of having to license your modifications under a GPL2-compatible license, which includes distributing source code together with binary builds.
And... I don't believe I see any links to the source code used by this project. Which I will assume until given reason to think otherwise, is merely a technical oversight.