This is a moment of history that is frequently referred to as a serious loss of collective knowledge up to that point. Is there any legitimate speculation and/or theories as to what kind of knowledge exactly was lost?
Nothing of value. That library in the Nile Delta has an overblown place in popular imagination. I commend to your attention this answer from u/XenophonTheAthenian, and this one from u/Naugrith, both of which expound on The Library.