Raspberry Pi 4: direct boot of IPFire kernel?

From what I understand, the Raspian kernel can be booted directly (kernel=kernel8.img in config.txt). Can something similar be done with IPFire?

I tried “kernel=vmlinuz-6.1.61-ipfire” and “initramfs initramfs-6.1.61-ipfire.img followkernel” in config.txt, but that just produced the square rainbow on the HDMI screen.

I don’t see this option mentioned in the documentation.