No manufacturer will tell you when the mechanical disk brakes are being used. The goal is to make a hybrid seem like a normal car behaves for braking and overcome the well known strange brake blending in hybrids. To do otherwise makes hybrids appear weirder and will cost sales.
Why does it matter when? You already know how to drive the car for greatest efficiency. If you do drive this way, physical brake use is minimized automatically. This is proven by how long the brake pads last on other hybrids. You can do this on manual shift cars as well. It was 140,000 miles on my last car before I had to replace pads. But if you are into jerky driving, your pads will not last as long.
I suppose it's the geek in me. Always wanting to know how and why. I could see KIA starting friction braking as soon as you touch the brake, all the way to only using them when regen is exhausted. The brake light is similar; is it certain regen braking, brake pedal, brake hydraulics pressurized, certain deceleration rate, some combination or what?
Does it matter? To my wife, for whom I bought the car; absolutely not, but to an auto enthusiast on a car forum, it does.
The marketing aspects, specifically making the car seem as "normal" with respect to the purely IC engine car; I understand but I don't have to like it. It just seems illogical but for one small consideration.......people don't buy weird cars.
On the other hand, it's all software anymore so why not? Coasting is more efficient than regen, regen is more efficient than friction so why not let the driver use that if he wants.
The pure EV's are going that way in all sorts of ways