I think that means that uboot can load from USB when the phone is connected to a host computer (and the host computer is running uuu) i.e. serial downloader mode.
I don’t think uboot can load from USB when a client device such as a disk presenting as a USB mass storage class device is connected to the USB port and hence also can’t load from the uSD card because that shows up as a USB device (i.e. a USB-based card reader internally)
e.g. as discussed here Librem5 sd and boot order - #16 by dos
So e.g. either a USB flash drive or a USB portable drive would be ruled out for now.