The AMI UEFI firmware behaves like most UEFI firmwares these days and locks out writes to firmware flash after boot and only allows updates by way of efi files being dropped into place and then updated via reboot with signature check.

The coreboot setup leaves writes to the firmware flash open and root can use flashrom at anytime.

This makes for security trade-offs, a choice between freedom to change your own firmware and know what you're running vs a mechanism to reduce the chance of advanced persistent threat at the firmware level.

Also, their build to order model under additional OSes does include Windows 10 as an option at an additional cost, so I imagine a UEFI firmware is required.

That’s my best guess.