If a car has enough torque and the RPMs stay high enough then it can start itself in motion without giving it gas. I regularly let the clutch out a bit to move out of the garage when the RPMs are still high while the car is warming up in the morning, and I don't normally have to give it gas, or much gas, to get it going. It shouldn't exactly be happy to do it, and you should get the RPMs up high enough so that you don't lug the engine when it starts moving, but I wouldn't worry about what you're seeing.