I'm not saying you're wrong, but how do you know?
I don't know. I'm guessing, but here's why. First off....
https://www.bluetooth.org/en-us/Documents/Bluetooth4-2FAQ.pdf
So now you know nothing hardware wise has to be done to be labeled as BT 4.2. So if the Bluetooth stack can be updated for compliance why would they update the hardware on older devices?
The iPhone 6 and 6+, both are still using an older Murato wifi/bt module then the launch day iPad Air 2 but they are 4.2? I think it's pretty clear Apple just updated the Bluetooth stack with iOS 9 for compliancy.
Then we have the elephant in the room, no differentiation in model numbers for different hardware.
Why would the refurb models still be Bluetooth 4.0 then?!? Because they are....the Bluetooth stack hasn't been updated yet because they are still shipping with iOS 8 like many mention on this forum. You have to buy new from Apple to get iOS 9 out of the box...even many new ones from box stores are still shipping with iOS 8.
https://forums.macrumors.com/threads/ipad-air-2-ios-9.1935719/#post-22209278
Like I said, I'm just guessing. But if someone asked me, do you think Apple retooled their assembly lines, remanufactured the logic boards with updated components for older devices, then stuck them into circulation with no differentiation from the previous model OR just updated the Bluetooth stack, the answer (for me) is pretty obvious.