Just looking at the third link, the most upvoted answer agrees that humanity suffered a significant loss of important information. And the 'myth' is just an asinine distinction regarding whether loss was due literally due to fire, or whether the information was lost due to some other cause. I think declaring it a myth in a conversation like this misses the point (it certainly isn't a distinction relevant to the original comparison made here to Yahoo Groups) and just serves to confuse people.