The DMG boot ROM fits into the top 0x100 bytes. To protect it from being snooped, the last instruction shuts off the boot ROM by flipping a register that can’t be flipped back, mapping the top 0x100 back to the cartridge (and allowing the cartridges interrupt vectors to be installed.) So, there is no jump to 0x100 at any point; they just fit the DMG boots ROM perfectly into 0x100 bytes and control flow leaves bootstrap right as PC hits 0x100.
This simple security mechanism was extremely effective, and lead to the DMG BIOS being undumped until relatively recently, when it was dumped by imaging the ROM directly and via fault injection to skip the last instruction.