It is because the China variants run a more customized flavor of Android. They do not run Android One, as is the case with global (WW) variants of the phone.
So, they are neither allowed to take part in Nokia beta labs, nor the Android P developer preview. You can decide to follow the XDA guide at your own risk :)
You don't need to update to P to have android one. All you need to do is flash the roll back zip then you will stay at android 8.1 official and receive google updates.