You're not wrong but how much does not precisely knowing the wheel's radius change things?

The operator would also experience an incorrect speedometer. For the purposes of a brake light it'd be off (either lighting up too soon or too late) by some amount but I imagine it'd be "close enough" except for the most pathological cases of tire sizes.

ABS relies on the fact that the computer knows how fast the wheels are turning in relation to each other. Wouldn't seem to be much of a leap to leverage that system to determine if the car is slowing down.

You don't need to know the wheel radius, you just need to the rate of change of the wheels RPM. Then the brake light could be set to some conservative value where regardless of any realistic tire size/inflation scenario the brakes will activate at a reasonable deceleration.

In any case, with standard gas/brake pedals on ICEs, I can barely tap the brakes and have my brake lights turn on without any deceleration on my part, or I can do that and still be pressing the gas and so actually be accelerating with my brake lights on.

If the car doesn't know the size and speed of the tires it also can't accurately display the current speed, which seems like at least as pressing a problem.

Edit: Or is detecting deceleration more sensitive? I guess I don't know how precise it needs to be relative to how precise a speedometer needs to be.

This is not a problem. We know this isn't a problem, because ABS almost always uses wheel speed sensors to do it's thing, and ESC usually taps into the same wheel speed sensors. Hell, those sensors are often used to tell you when your tires are low on pressure, and yet that doesn't prevent any of the other functions from working, or even affect your speedometer enough to care.

Changing wheel size enough to affect readings of speed does not change the broad slope of the derivative of wheel speed.