Depends on the system. This definately holds true on the cheaper/less sophisticated torque sensors.never hop on a bike with a torque sensor without grabbing the brake first or make sure you dont touch the pedals when hopping on, any pedal pressure can and will cause the bike to go!
It doesnt not hold true for any of the more upscale mid drives I have used (Yamaha, Brose(I own 2), Shimano, Bosch). None of these have brake cutouts.
My cheaper Juiced CCX torque sensor definately can take off.
That being said, its a good idea (on any bike, even non electric) to apply brakes when getting on a bike. I have been doing it for years