@Walter_vi_Britannia So it seemed. But if you re-read the novel, the author actually hinted that their relationship is more than that. Them sharing the share computer (with half windowscreen, like the author did during his date with his wife), her sleeping on his shoulder, accompanying her eating ice-cream, several characters hinted that Su Mucheng is his girlfriend, and the part where Ye Xiu told Su Mucheng to keep in touch after his retirement and that he will wait for her to retire (he only said this to her and no one else). Also, Su Mucheng's thoughts are full of him.
Su Mucheng is Ye Xiu's most popular bg cp, mostly because she is waifu material and the only one that made sense due to how their relationship is portrayed throughout the novel. Fans preferred Chen Guo with Wei Chen instead. As for where I heard that, it is from the Chinese fans. The author mentioned it either on social media or fan interactions. He is not good at writing romance, so most readers did not notice the subtle hints.
There are other ships too. Du Ming x Tang Rou and Xiao Shiqin x Dai Yanqi. Subtle.