Apple didn't have to release the source of Webkit from 2003 until December 2005 because they didn't distribute any binaries. The GPL does not force you to release any source code as long as you don't distribute any part of the work. If you're only making changes and improvements, nothing forces you to release those until you're ready to ship your fork.
KHTML is not always KHTML, in fact, the KDE project had a big beef with Apple when Apple finally released Webkit and their modifications were close to impossible to merge back into KHTML.
And yes, Apple needed to release Webkit as open source, at the very least, the libraries which were forks of LGPL code. Otherwise they could not distribute any works based on Webkit (including never releasing Safari).
I think you need to read up some on how all this open source licensing work, you seem to be missing a few points.