I'm not familiar with the Debussy prelude 12, but more in general, you don't "need" to alternate fingers for repeated notes, but in the end, when you've mastered the technique of alternating, you can not only play them more even but also quicker. That's often what we are talking about when talking about 'repeated' notes', it's getting them repeated in a brilliant, pearly and speedy way...
Kindly.