Changing the timing belt requires removal of the crankshaft pulley. This requires HUGE torque to re-install, and there is a Toyota special tool needed to hold the crankshaft while the pulley bolt is being torqued.
If the crankshaft pulley isn't reinstalled properly, it will eventually work loose and damage the keyway in the crankshaft nose, requiring that the crank be replaced. This situation isn't uncommon - use the forum search function and note that in some instances a Toyota dealer had done the timing belt change that preceded the pulley coming loose. It's obvious that you can't just use an impact wrench to tighten the pulley nut and get a safe and dependable outcome.
If it were my truck, I'd inquire of the service manager whether or not they have the special tool and a torque wrench that'll handle the 200+ lb/ft of torque needed to do the job right. If the answer to both these questions isn't a resolute "of course", I'd not let them do the job.