I want to note that the XK-1(A) does not use the BOOTROM flash loader but a custom loader in the OTP.
I am not sure whether it behaves different.
Everything always uses the boot ROM. If the "boot from OTP" bit is set, the
boot ROM code reads an image from OTP to RAM, and jumps there.
The image in the XK-1 checks if the mode pins are set for SPI ROM boot,
and if not, jumps back into the boot ROM, which well then boot from whatever
you have the MODE pins set for.
(And if the MODE pins are set for SPI ROM boot, it will run code that is
essentially identical to the boot ROM code for that, just different ports,
and it checks for an image that is too big (unlike the boot ROM code)).