I agree with your later comment, that it's unclear whether ultrasonic infection was actually claimed. For purposes of this thread, which started by trying to refute the position, "BadBios is totally impossible" -- the "impossible" claim the mainstream is probably referring to is infection purely by ultrasonic transmission. There's no need to prove that ultrasonic communication is plausible, or that rootkits can be installed in BIOS, EFI, harddrive controller, etc. It is impressive to see such exotic attacks actually being used, though.