Of course she was. Putting your car into reverse and letting it back fully into a pond without braking when you actually meant to put it in drive is not something an unimpaired driver does.
Eh, not defending a drunk driver but there's more to this:
the Tesla gear interface is a touchscreen where there's no "feel" for which gear you've selected if you fatfinger it.
many folks, sober or drunk, when they get the car moving in an unexpected way reflexively stamp down on the pedal their foot is currently on. This could happen to you. You are not immune.
So if you are in the wrong gear, then panic stomp the pedal, you can absolutely find yourself speeding in the wrong direction.
Adrenaline does weird things to your brain, when controlling a car. When startled, we default to getting low and stable. That's good when standing, however, it putting your feet flat on the ground is extremely bad when in the driver's seat.
I've actually witnessed this in action. A slip resulted in 2 cars written off, 1 pushed through several gardens. I can easily see someone doing something similar, with a Tesla. Alcohol would just amplify the effect.