IIRC, Moffat was also cagey about who John Hurt's character was in 2013.
However, maybe Russell wants to explore the idea that maybe regeneration isn't what fandom has long assumed it's been. The nature of regeneration developed across the original series -- a renewal by the TARDIS, something forced by the Time Lords as punishment, weird Buddhist things, etc. -- and the idea that it's a simple "a Time Lord's whole body changes when dying and close to death" was a relatively late development ("Time and the Rani"). And the last three regenerations have been weird -- the clothes change, the bigeneration, Billie Piper.
Maybe the Doctor didn't regenerate into someone who looks like Rose. Maybe he regenerated into literally Rose by pulling her out of her timestream, and regeneration is a whole hell of a lot weirder than we thought.