Running the truck in the air only applies drive train drag to the motor - relatively unloaded. The 20A current you see is probably just what the motor is pulling to get the tires (which are just acting kinda like flywheels) up to speed.
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22N03S FETs in the older versions of the MM are rated for 50A each @ 77*F, there are 6 in parallel at any one time giving you a 300A rating. Even derating to 50% of that value for ~170*F is still 150A. Of course, there is more to it than just the current rating (like power dissipation), but I would be more concerned about the PCB at that point. The Quark 125B uses the same FETs (and the same number of them), and that is rated for 125A. So, don't be too concerned with keeping it under 100A. Besides, those high current peaks are for a very short time; the average current is much much lower.