The operative phrase is meeting very openly. It is not Christianity that is illegal. Church bells ring a half block off of People's Square in Shanghai to announce the start of church service and people flood into the church. This is just as they do in churches all over China.
What is illegal is unauthorized assembly and what is referred to as underground churches. Just as this article states. It is the same reason Amway ran into trouble. Amway is not illegal in China but its motivational private meetings in people's homes or store fronts is illegal.