"Apple could have kept both closed and innovated on them"
Webkit is based on GPL code from KDE, so Apple had no choice but to release the source.
Webkit is based on GPL code from KDE, so Apple had no choice but to release the source.
Originally they just threw useless code patches back at KHTML to minimally comply with the license (LGPL, iirc).
Remember, Apple had experience web engine developers on the team from day 1. They knew exactly what was needed and how it should be done.