One could say that the orphanage portion was a coincidence. However, when Xu Yue'er was alive, she was always bragging to her neighbours that she had a son who had grown up in a big official's family and was someone big himself now as well while her grandsons and granddaughters were extremely competent.
That was clearly no coincidence then.
All her neighbours thought that she was spewing bullshit but in reality, she was referring to Su Ya's family.
Besides, 50 years ago, women would be sleeping in a common ward that consists of 4-6 beds after giving birth.
Of course, if they were in the same ward, one could say that carrying away someone's child was a mistake.
But if they were in different wards? How could there be a mistake, especially when the child hadn't even been carried out.
Lining everything up together, it was easy to deduce that there was no mistake that those two were mixed up in the past.