If any segments of the highway drive are downhill, you'll recapture some energy even on cruise at constant speed.

It has to be a quite noticeable downhill for a car to net pick up energy at highway speed. Small downhills still require positive power consumption (to overcome rolling and aerodynamic drag).

Obviously, there’s a point at which a downslope will offer regen, but these are the slopes where a manual transmission car would pickup speed if placed into neutral.

That's not my experience in my renault zoe. Often on almost imperceptible downhill slopes it will be at zero consumption, and there have been many areas where I can't back off the throttle in my ice vehicles where the zoe will be regenning.

I find similar at low speeds around town, but once at highway speeds (70-75 mph) in my LEAF, the aero drag means almost imperceptible downhills aren’t going to let the car accelerate with zero consumption.

Any downhill steep enough to make you drop gear or hit the breaks in an ICE is a point where an EV would regain energy.

True.

I'm assuming 22% is meant to indicate the average based on aggregated driving conditions.

That is true for a good flat one.

On one that curves and bends up and down, it is possible to capture back some energy, though, you do have to spend the energy required to go from point A to point B.