People say this without any evidence. This ai-post is just regurgitating hn-thread "received wisdom". The evidence for the existence of a library is thin and hard to piece together, but points to more than a myth. I appreciate that people want real proof of anything, but dumping an ai-slop summary is hardly doing any better than accepting the existence of a large library.
The Library almost certainly existed. It is the destruction (by deliberate fire) that is probably myth.