Repeatedly over the years, Gov. Tim Walz of Minnesota has said that the year he spent teaching in China began with a trip to Hong Kong during the pro-democracy protests in the spring of 1989 that culminated in the deadly crackdown that June in Tiananmen Square. As recently as February, Mr. Walz said on a podcast that he had been in Hong Kong, then a British colony, “on June 4 when Tiananmen happened,” and decided to cross into mainland China to take up his teaching duties even though many...