I agree with that, as the coefficient of friction between the tire's contact patch and the road surface is a variable, and that very aggressive braking will alter that state dramatically. Temperature rise inside the tire & the road surface play a very important roleThe main stopping benefit of ABS brakes is to avoid the steep temperature rise at the skidding footprint, which softens the rubber and allows it to slough off, thus lubricating the interface, leaving the skid mark and lowering the available stopping force. On ice, the reverse occurs; the surface of the ice under the tire melts in a skid, also best avoided for the shortest stop. On gravel ABS makes little difference, except maintenance of control (important in all three situations).
At least that's how my aircraft maintenance instructor explained it all those years ago.
Just consider the brake pads & rotor on that wheel in two different cases. In a static condition with the car at rest, the brakes applied to a preset hydraulic pressure, and with a long lever temporarily attached to the wheel - measure the amount of torque it takes to get the brake rotor to slip across the brake pads holding it in place. Then compare that torque to what is available in a kinetic case, where the car is moving before the brakes are applied. That adds a thermal effect to both the brake rotor and the pads. The pad material will exhibit very interesting behaviors, one of which is the generation of gases that act to lubricate the contact point between the pads and the rotor. So the standard rotor should have two versions, one a plain rotor and another as a vented type. My point though, is that you will always find a much higher braking force is possible when the car is at rest before applying the brakes
If for example, a tilt table were to be built, large enough to carry the car in question. And that this table were to be covered with the same exact material as used on a test track located at the same facility. And then install in the test car a simple pendulum with a damping mechanism to steady it's position. Park the car on top of the tilting table and while holding the brakes at a set pressure, tilt the table forwards until the car slips (safety straps would be used to limit the slip to a safe distance, say 4") Now take this car out on the test track and perform a stopping distance test. No matter how small you can get that stopping distance, the pendulum will never reach as high up it's scale as when the car was on the tilt table. This is in my opinion mostly due to the effects of thermal rise at various points, which can be viewed in a simple manner as the exchange of energy required to slow the car
But the OP wasn't asking for real-world conditions. I would only suggest adding the effects of temperature rise to the hypothetical 'rubber tire rolling down an incline' model