I found the answer:
https://www.raspberrypi.org/forums/viewtopic.php?f=63&t=289934&p=1755420#p1755428As you've surmised from the title, this question is pertinent to the AIO (All In One) Raspberry Pi 400 computer - the model where a higher clocked Pi 4 has been designed into a red + white keyboard casing as seen below, not any other model.

Does anyone here own the Raspberry Pi 400 (again, not the 4, the 400), as I hear it is missing the EEPROM which stores boot configuration info, where it is present on the Raspberry Pi 4 model. Does this affect the 400's ability to USB boot?
Many thanks. Please be as verbose as possible.