Not quite accurate...
Learjet Service Bulletin SB 60-78-7 modifies the aircraft to allow the T/Rs to receive an input from the wheel speed detect box (used for autospoilers). This would prevent an inadvertant stowing should the squat switch be damaged during landing/aborts.
The danger of an uncommanded stow following a squat switch failure is that the FADEC will initially command the engine to idle, then to whatever power setting is requested at the thrust levers after the T/Rs stow. The "piggy-back" levers, if still pulled aft, would be commanding a high power setting of the FADEC (which believes the aircraft is airborne and just stowed a T/R). I believe that the piggy-backs move the same linkage as the thrust levers do to a rotary switch that is interpreted as a thrust lever angle by the FADEC, thus giving up to nearly takeoff power while you tried to stop. This scenario would be good if you really had a T/R deploy while airborne, but can be horrible if it happened on the ground.
The AFM has a Emergency Procedure (as a Memory Item) that instructs you to close the piggy backs should a T/R stow by itself to prevent this situation
Also, the above SB did not apply to N999LJ based on serial number, so I assume it had it installed.