first of all the aux battery is not designed for cranking and the cca (current needed to crank the vehicle during winter/cold) is very low. the car you jumpstarted probably has a battery that is not completely dead but just partially drained. Note that in conventional cars/trucks, jumpstarting won't work if the battery is completely dead or shorted unless you have a large booster battery such as in tow trucks and dealerships.I'm curious why you can't? I was in our 09 TCH and jumped a guy in a Gmc Sierra that had left his lights on. Pretty straightforward.
on prius owner's manual, they don't recommend jumpstarting another car unless you disconnect the aux battery and use that to charge the dead battery.