The 10% over 4 years represents nearly 100% over a lifetime. Given the rate of conversion will drop year by year for the remaining population this still leaves a bare minimum of 50% conversion. Some missionaries estimate the rate of practicing Christians in China is more than 25%. It is especially notable that government repression of Christians and Christianity is rampant especially in rural communities.
If they keep practicing, they'll get it right eventually.
OK, I'll be Sirius. My fearless prediction is that within the next two generations China is going to trifurcate, with Christians dominating the east (joining spiritually with the Christians in what will certainly become a united Korea, and maybe even a mostly-Christian Taiwan), Buddhist/Taoists the center, and Muslims the west. It won't make for stability on the mainland, but a Christian Han China would be much more likely to ally with Japan, Viet Nam, the Philippines (unless the Muslims dominate there), and the US to make for a peaceful and prosperous Pacific region.
The number of Christians in HK is vastly underreported.
Example: The largest Protestant Church in HK is probably The Evangelical Community Church of Hong Kong. It has two location, one on HK Island and one on the Kowloon side. Each branch has multiple morning services with over 800 - 1000 attendees at each service yet the church had, in 2012 when I left HK. less than 500 actual members.
The reason for this is many Christians there have Buddhist parents who will object (A real source in multi-generational households) if their child formally converts by joining a church. So they attend church, tithe, lead Bible Studies and do all the things “Christians” do but do not affiliate with a church until their parents die.
China is the same way. As odd as it sounds, China may be the closest thing on Earth to a Christian nation at this point and time.
I listened to a Voice of the Martyrs pod cast once. A fellow named Ali was asked about his experiences sharing the Gospel in Iran. He claimed that it was thrilling because of the risk and asked that when we pray for closed countries that we pray that the countries will remain closed and that we should pray that those who share God’s message of salvation and peace be hidden from the eyes of the enemy. That is how I pray today. I guess that the light is brightest when the room is completely dark.