I don't think preparatory exercises are necessary. You learn to play the Etudes by playing the Etudes, not by playing other stuff.
Having myself worked extensively on the Etudes, which I started doing before I was ready, I have to say I disagree. I have wasted so much time relearning the Etudes because I first learned them when I was too tense and had too many bad habits.
IMO 15 mins of Chopin Etudes would be more beneficial than 15 mins of Hanon. I digress.
But there is a difference between learning to walk, then learning to run, then learning to run professionally, and trying to learn to run professionally before you can barely walk. The latter is likely to have disastrous results.
Don't you mean 15 minutes of Chopin Etudes would be better than 10 hours of Hanon.
But there is a difference between learning to walk, then learning to run, then learning to run professionally, and trying to learn to run professionally before you can barely walk.