Maybe they just do things in a different order? Arrange a wedding and then propose to seal a formality and for show. If it's China, I really wouldn't be surprised!
Someone said that the couple already registered a year ago. The wedding was obviously planned in advance. The "proposal" was probably a real romantic surprise to WX, just for effect. It's fairly common for Chinese athletes to hold their weddings after their retirement, years after getting married.
Wang Xin is returning to the national squad, as a coach. http://sports.sina.com.cn/o/2018-04-05/doc-ifysuuya7804815.shtml http://k.sina.com.cn/article_6484922113_182880f01001005afr.html?from=sports&subch=badminton
Wang Xin joined the CBA as chief coach for the national junior team, officially from March 2018. She has also completed her studies at the Beijing Sports University in 2017 after getting married in 2015 when her then boyfriend popped the question and the couple has a baby girl now.