AFIR, usb boot requires u-boot supporting a) usb initialization at boot time and loading kernel from USB in priority to other devices, though u-boot anyway should be on eMMC or SD (or spi flash when exists).
Priority of these is hardcoded in CPU's BootROM, which looks for loader (say u-boot, for simplicity), which in turn does all the job.
Setting up usb in armbian-install might only point u-boot to partition guid on usb rather than eMMC or SD (in armbianEnv.txt), but this is not the same as prioritizing USB over SD and eMMC.
To achieve USB load if USB flash is present, one needs to ensure that u-boot supports this. Further setup has nothing to do with armbian-install, just flashing image to USB is sufficient.
Although USB boot looks attractive, it actually requires enabling features, which slow down boot for several seconds and it will not help if u-boot at eMMC/SD/spi flash is damaged. So in fact it is just a toy
