Most ABS logic loops at their core involve activating the brakes until a slip is detected, reducing brake pressure until the slip stops, and repeating. For that reason you will be prone to losing traction in bad weather (higher risk road conditions) from moment to moment. More conservative logic predicts when traction will be lost and attempts to reduce brake pressure before that point has been reached. That means reducing stopping power for fear of losing it entirely. This is where downshifting can be a major improvement. It tends to net a better initial "bite" in bad weather because it doesn't pulse the stopping force and forfeit extra deceleration.

While your logic may seem sound at a high school level, the real conversation is a bit beyond your grasp of the physics involved. In the real world, the calculations on paper, however absolute they may appear to be, can often be proven wrong in testing due to factors we may not understand or previously predict. For example, you're right that brakes tend to outpace engine braking. That's a mathematical fact. However, once you cross the threshold of available traction, your tires slip and you suffer a massive loss in stopping force until traction is restored. Brakes push you to that threshold rather quickly if the driver misjudges how far they need to push the pedal. Downshifting tends to dance just short of that threshold, maintaining traction from start to finish. 5 seconds of moderate deceleration from downshifting can net a quicker stop than 2 seconds of intense deceleration and 3 seconds of skidding when the brakes bite too hard. On paper, you'd find the brakes to be the clear choice. Yet in the real world, the downshifting can produce a better result.