It is certainly true that this question has been discussed here extensively in the past. What is somewhat new is that the producer, Robyn Goodman, was quoted in last Sunday's long article in the NYT by Patrick Healy on the impact of previews on Cinderella and Kinky Boots (which I've linked) as saying that she thought Cinderella should have been nominated as a new musical in spite of the "classic" rule. Having read the text of the rule, I'd suggest that as strong a case could have been made for Cinderella to be "new" than a revival, given that the show a) had never before been on a Broadway stage;and b) is truly a completely re-written work. It's hard to see the show with Beane's book and think of the show as a classic.
So, it was simply left to the whim of the Tony Committee, and they are neither "right" nor "wrong", it could have been either.
Personally, I would have put it into the "new" category.
Healy article
CZJ at opening night party for A Little Night Music, Dec 13, 2009.