I think its because we don't actually know when they started dating. They said they weren't dating long, but......they were already having sex when WQ found out, and clearly he was the last to know about their relationship so....Not to mention she said she pursued him, and he said he spent time thinking about dating her before they started dating. I guess it depends on what they view as 'not dating long'?
ETA: reading the book. Assuming the age gap is the same in the drama, even if they changed the ages. Lili was 5-6 and WQ was 13-14 when their mother died. SP age is stated at 3-4 years older than WQ....so at best the age gap between Lili and SP is 10 years and at worst 13 years. Also, in the book, WZY told WQ when he took him in that he was 8 years old, but WQ thought he was likely not telling the truth and was closer to 6-7 when WQ was 14.