I don't see why addons should need to be able to cancel auras/buffs.
What's troublesome is that the green posts linked on the wow forum don't seem very clear as to whether buff addons will still be able to present right-click-to-cancel functionality for auras/buffs gained in combat. Or maybe it is clear - I don't know much about secure button templates or whatever.
You wondered why they now use CancelUnitAura instead of CancelPlayerAura. Well, I think it has to do with the new vehicles since they also use PlayerFrame.unit. So most likely there will be a change in the unit you control when using a vehicle I think.
Vehicles are considered pets, yea. The default UI swaps the player and pet units on the frames. If you use a UF addons, the vehicle shows up as your pet. So it makes sense that if you're in a vehicle and can cancel special buffs, you're not cancelling them from you, but rather from your pet.