As someone else noted, this is due to the issue solved with GPL3, which redefined the roll of derived work and "linking": as it stands, WebKit *is replaceable,

As someone else noted, this is due to the issue solved with GPL3, which redefined the roll of derived work and "linking": as it stands, WebKit *is replaceable, but doing so requires you to first defeat the cryptographic locks that prevent you from modifying any of the software at all.