OK, I get that part of it now. There's no issue with the U-Boot image. I wasn't waiting long enough to get the new u-boot prompt. I can now write the partition table and download images to the board. Sorry for the noise on that part.
Once Debian boots (so far, so good), I dd the bootloader.bin and u-boot-dtb.img (I copied them over from my host) using steps 13 & 14. Unfortunately I don't get a bootable system at the end. Are there any gotchas I could've missed in the last 2 steps?