I don't believe this to be true, and I don't know where this comes from. Their documented GPLv3 reluctance predated code signing on their platforms by many years. FWIW, GPLv3 was controversial even amongst GPLv2 advocates for all sorts of reasons, never mind amongst actual lawyers. No conspiracy theories required, even if you disagree with their position. The Linux kernel and many other non-GNU projects never updated to GPLv3 and it had nothing to do with enforcing anyone's walled gardens.