It is not a first gen product.
Examples of True First gen products:
1. Original iPhone 1
2. iPad 1
3. Apple Watch
4. First redesigned Intel iMac
5. Magic Mouse 1 and trackpad
6. Magic Pencil
iPad Air Pro is just an evolution of iPad Air 2 with incremental upgrades to internal and external components proportional to size increase. It has the same Arm based CPU and PowerVR based GPU architecture.
Usually when Apple updates one component for example screen resolution or pixel count, they deliberately leave out some other area like CPU/GPU. This very fact causes uproar. But this time with iPad Pro, Apple have learnt their leason and upgraded all components within reasonable proportions. Be it RAM, speakers, battery, CPU and GPU.
What we are left with is a tablet which performs exactly same as iPad Air 2 and no iOS feature which leverages it's upgraded hardware. Yeah few tabs and apps reloading sure!
So we can conclude that this iPad Pro is NOT a first gen product. Even Apple's internal numbering iPad 6,8 or (6.8) is a proof of this.