The changes made were not as simple as not-setting a lock fuse bit. Making the bootloader unlockable in a way that didn't compromise their existing security mod

The changes made were not as simple as not-setting a lock fuse bit. Making the bootloader unlockable in a way that didn't compromise their existing security model did require going out of their way. The status-quo for previous "apple silicon" bootchains (iphone, ipad, etc.) was not built this way. Even T2 macs had no way to boot custom firmware on the T2 chip, without exploits. Sure, they could've done way more, but evidently they'd rather not lock down the platform completely.