I do not know the cause of this accident but your assumption is wrong.
"mechanical jam (as crew describes) preventing the throttle levers from moving into reverse detent (which is apparently required for automatic braking/spoiler operation)"
Autobrakes and auto spoilers are in no way dependent on reversers operating although deployment of the reversers will trigger spoiler deployment whether they are armed or not.
Statement is true in itself, however other single source failures can result in these events being concurrent. A simple trip/failure of the G/A sensing relays will result in reverser interlocks being inop, brake pressure being limited to IFB, and autospoilers not functioning, or combinations dependent on the aircraft type. On some aircraft, the manual spoiler will be limited to flight values, not ground operation values.
Does this occur? occasionally, even on an A340 having a hard landing event
