It should be better than the mockup
It seems to me that if Apple chose to leave Siri off the new iPad during its release it was because they wanted a refined way of presenting the UI specifically on that platform and since iOS 6 was already in the works at the time, it makes sense that the major effort would have been spent during that development cycle to make it happen there for the iPad form factor (not backport something to iOS 5 that was nearing EOL).
Now, if Apple spent all that time refining the iPad version of the SIRI interface in iOS 6 (and disappointing the new iPad adopters hoping it would be there in the process) only to have it still look like some iPhone-sized, shoe-horned in interface, that would be somewhat foolish on their part. They could have implemented that mockup interface with minimum effort into iOS 5 well before the iPad launch.
Needless to say, I'm hoping for something more impressive than that mockup.