The reason for the underestimate is that many Christians in China are unaffiliated with any official church. The government doesn't allow more than 25 people to meet in a gathering without official permission, so Christians in China have formed thousands of "house churches," similar to the way Pentecostalism has grown through small groups across South Korea. Just as private meetings in homes were how Christianity spread in Rome, the result has been a proliferation of more and more smaller groups in China. These house churches tend to be unaffiliated with any denomination, with "no fixed liturgy or tradition," and run by recent converts who themselves are not expert on Christianity.
The article points out that in China, Christianity is not associated with tradition and ritual, but with modernity, business and the market economy (where it's seen as a necessary check on the market to provide ethics in business), and science. Six of the 30 leaders of the student protests at Tiananmen Square became Christians, and the article states that "One Confucian Chinese says with a rueful smile that most of the pretty girls at university were Christians--and would date only other Christians."
Christianity has spread in China as a result of rural Christians migrating to the cities, and students who travel to the United States and become Christians and then return home.
I look forward to seeing a similar article in the future about the spread of atheism, skepticism, and pro-science groups in the United States through Meetup groups and student groups.