I don't dispute that people get used to driving, but I think arguing that "driving" requires the same amount of effort as "not driving" is a more specious claim
It's really not, the human brain is _really_ good at abstracting.
In pretty much exactly the same way you don't have to pay attention to balancing to walk, your brain learns to do the same thing for cycling and driving. It abstracts the mechanics of the car (e.g. turning the wheel to turn left/right) and sort of extends its view of your "body" to the car.
This isn't an instantaneous process of course, it takes time to build up that abstraction, but it absolutely exists. If your brain wasn't good at doing those types of abstractions then walking/running/typing/speaking/breathing/etc. would take a phenomenal amount of attention and practice.
For some people it’s easier to zone out and drive on auto, for others its easier to use FSD. Some people find it easy to zone out in bumper to bumper traffic, others not so much.
Except by your own admission you're not "not driving." You're monitoring the car, except without any physical feedback loop with your hands and feet. The only difference in mental load is the physical control of your limbs to do minor speed and direction adjustments. YMMV but I just find that less taxing than vigilantly monitoring a piece of software because I know how that sausage is made.