That would be a hilarious oversight - and therefore a totally Toyota thing to do - to just run a 1.5kW inverter off the little 12V battery
They probably actually limit the output, if for whatever reason they're unable/unwilling to top up the 12V with the HV battery. One hopes!
edit: ok a quick google search says you have to have the car on "Ready" to enable that AC outlet.
in my FJ, it was limited output when in accessory mode, and the full power when you had the vehicle running.