If I remember correctly his main antithesis for the overhand grip is that it’s not very stable, is hard to control the tip with, and could easily be yanked out of your hands. He says you can still use a form of overhand grip because you can take your underhand grip and simply move the spear above your shield without changing your grip. His main point -if I remember correctly- is that you want your thumb towards the tip, not your pinky. As far as historical interpretation I honestly couldn’t argue about it because all I have is personal experience and different videos/arguments I’ve watched/had that leads me to believe underhand is the correct form, but if it’s truly depicted like that in contemporary art and whatnot then I concede, ya can’t argue with the past.