Maybe he sees him as a soft or slow-developing horse, and doesn't want to have the arse torn out of him by a tip-top hurdler.
Ruby would be much the best guide to a horse's ability and preferences - exactly the same as he was at Ditcheat.
Look at Hurricane flys career, he was straight into Grade 1's, Mullins knows when he has a good one and based on how he's campaigning UDS he mustn't think he's up to it.
Wasn't Thousand Stars the only 'Grade 1' in the yard when the Fly came along?
I think the presence of the Fly - several Grade 1s per season in the bank and as strong a challenger for the Cheltenham race as you'll get - allowed him to take things slower or go down different routes with the others.
Thousand Stars didn't win a Grade 1 until June 2011