Why is this questionable?
Look, there is nothing spectacular about the iPad. People seem to think its an engineering marvel when iPad's are made thinner.
When the iPad 2 came out everyone was all impressed about how thin they made it, yet the iPad 2 contained all the same components as an iPod Touch and the iPod touch was thinner. iPad's have a lot of space in them compared to what you find in an iPhone and iPod.
It stands to reason that iPad's should always be thinner than the iPhone. If the iPhone 6 is as thin or thinner than current generation iPad's then it stands to reason that next gen iPads will be thinner.
It's not an engineering marvel to reduce the thickness of something with like 200%-400% more volume than a smaller product that has to pack all the same components in it. Sure an iPad needs a larger battery because of the larger screen, but considering that an iPhone is mostly battery these days the iPad just has naturally more room for a larger battery without getting thicker.
The iPad is just an iPhone with a large screen, if they end up being thicker than an iPhone 6, this is actually just lazy industrial design.