Question [Q] Replacing bootloader

I found the following U-Boot port for Galaxy Nexus:

github . com / Ksys-labs / uboot-tuna

But when I try to flash the bootloader via fastboot, the device refuses to accept it because the magic number in the header is different. Makes sense. But according to the docs of Ksys-labs, this bootloader can replace Samsung's stock bootloader and there's even a video on Youtube which claims to demostrate a device booting with this bootloader instead of Samsung's.

How do I make the device (maguro) safely accept the U-Boot bootloader like they did? (I asked them, no respose so far)