Hi Patricia,
It does seem a little contradictory, doesn't it? But I think Chang's point is that scale playing actually develops lots of different technical elements at the same time (finger independence, touch, phrasing, thumb over, etc). And since scales are a major building block of Western music, if you can play them well, you make your life easier every time you encounter a scalar passage in real music.
The repetitive activity he is against is the Hanon style, which he says simplifies technique to an unrealistic degree. What I took from Chang is that scale playing should not be mindlessly repetitive, with both hands together going up and down...up and down. Instead, vary how you practise them and be aware of what you are trying to achieve at all times.
So practising hands separately is a major idea, only putting hands together to prove that you can coordinate them. Another suggestion he advocates is playing slowly and deep in the keybed at first.
And the one which really helps to play scales faster is to play the scale as two "chords", e.g. for the right hand, C major is C-D-E and F-G-A-B. Since there's always a 3-group (A) and 4-group (B), you find and "see" the notes easier that way. So you start by playing them as chords (A group, B group, A, B, up the octaves and down) and instead of thinking of scales as seven notes, you think of them as these groups. Then you try "rippling" the notes fast, hit the transition between A-group and B-group and, hey presto, fast scales.
No doubt others with more classical experience than me will chime in at this point. But I hope this gives you a basic answer.
Topic: repetitive practising of scales