GPLv3 added language which specifically requires that GPLv3 software embedded in products be distributed with "Installation Information" explaining how modified versions of that software can be installed. It is not at all clear how this would apply to software in an immutable, signed system volume. This language was not present in GPLv2.
https://www.gnu.org/licenses/gpl-3.0.en.html#section6
(There are other issues with GPLv3 which make it a dubious choice for companies like Apple, but this one is particularly problematic.)