Turning the fuel off seems roughly equivalent to turning the ignition off when you've parked your car. It's really something rather unlikely to do as a brain fart during takeoff.
But we can already conclude that something unlikely did in fact happen, otherwise there were more fatal 787 crashes. I don't think "it's unlikely" is a good argument for dismissing potential causes here.
They do make an effort to make it hard to do safety critical stuff by accident though in cars, small planes and jets. Like in a car it's easy to mix indicators and windscreen wipers which are on various stalks but turning off the engine and locking the steering is very different action, on traditional cars turning the key and taking it out. Similarly here the fuel cut offs are obvious levers that have to be pulled out before they can be moved.
You have to lift and turn these knobs. Hard pressed to think of other knobs like this one. Especially that there are none like it anywhere near those cutoff switches. It seems intentionally designed this way
