Is “Is it a historical fact that Sun Ce captured Lujiang and defeated Liu Xun?” true?

It indeed happened. The official history, Records of the Three Kingdoms, in the biography of Sun Po Lu Tao Ni, records that Sun Ce tricked Liu Xun into attacking Shangliao, and "as soon as Xun set out, Ce with light troops attacked and captured Lujiang at dawn; Xun's troops all surrendered, and Xun alone with a few hundred of his followers returned to Cao Gong." The narrative in Chapter 29 of Romance of the Three Kingdoms, "In the fourth year of Jian'an, he captured Lujiang and defeated Liu Xun," is highly consistent with the official history, and the novel follows history here.