USB device mode on usb0 is on when boot into kernel. Some custom boards do not have usb0 as device mode so it will fail.
It also needs your kernel be able to detect the nvme drive. For example, if you didn’t use the default pcie for nvme as devkit and changes it to something else, then default jetpack won’t be able to find this nvme so flash will fail too.
That section is talking about how to escape from such case.