The braille example you give makes absolutely perfect sense. The blind student is being evaluated same as the other students and the accommodation given to the blind student (a Braille version of the test) would be of no use to the other students.

But extra test time is fundamentally different, as it would be of value to anyone taking the test.

If getting the problems in Braille helps the student demonstrate their ability to do Calculus, we give them the test in Braille. If getting 30 minutes of extra time helps all students demonstrate their ability to do calculus, why don't we just give it to all students then?