I also saw that Xunlong created a Linux 6.6 branch for that kernel. I hope this also means they updated their bootloader source, or there is a mainline-based U-Boot source.
The source you linked does not include a bootloader at all, hence seems to work with the present SPI bootloader only, and the SPI storage is optional on that board, AFAIK. The README mentioned a U-Boot patch dir which does not exist 
. Anyway, a better starting point.
I will play with it this weekend.