With the autopilot off, the autothrottle goes off at 5’.
Interesting to know. There is certainly nothing in maintenance manuals about this.
I doubt the thrust levers would go into idle, however. Without cues from the FMC or A/P, the A/T must surely go into a hold mode for safety. Having said that, the 737NG has a flare thrust lever retard mode at 27' RA if the A/P is not engaged (with the aircraft on the ILS and flaps extended). The NG A/T disconnects 2 seconds after touchdown.
I assume in the absence of a valid Rad Alt signal, the A/T would remain in SPD mode.