They are. For example, in this[0] crash report (found with a quick Google search), on line 77, it clearly shows that the WAKWindow function is inside the WebCore binary (statically linked). You can also find this by running class-dump on a copy of WebCore from iOS: there are many linked-in WAK classes do not have source code released as the GPL requires.

[0]: http://pastebin.com/GvFVfWHV