Maybe, but I doubt it. The vast majority of the sources, including the two listed for this claim, are listed as "attributed", meaning "A model (Opus or Fable) claims that this is what the cited source says. In my spot checks, I've found these claims to be accurate; still, since it's just a model claiming that author x says y, there's a nonzero chance that it's a hallucination."[1]
I couldn't track down the first source, but the second source goes over something completely different. Maybe Claude accurately remembered the first source when it was stolen during the training process, but forgive me if I'm skeptical. Feel free to track it down yourself and let me know. Otherwise, I will assume it is an LLM hallucination.
(Also, given my background knowledge of the Tang dynasty, I would be surprised if the average marriage age was as high as 18!)