Well, they are two quite different things:
- the USB can be used to get the initramfs running by the u-boot bootloader, which takes care of the really early stages of booting to get the proper Linux OS (OpenWrt or OEM firmware) running.
- after Linux has been booted up, u-boot and its driver capabilities have no role. Loading image from USB in OpenWrt requires now USB storage support in OpenWrt.
In general, the OpenWrt device wiki pages do not each try to explain basic Linux file copying over network, drive usage etc. Otherwise, each wiki page would be much longer. The wiki pages assume that the user has basic knowledge Linux console tools, (and hopefully even understands the role of bootloader and the actual OS).