"E.g. Did Leif Erikson even exist? The evidence is pretty flimsy"
Did George Washington exist? Unlike Lenin's, his body isn't on display for our viewing. The book is a great big load of crap, and saying that (or saying it again) doesn't diminish me, it merely points out that the book is great big load of crap.
Ever hear of Gilbert Stuart? Maybe he didn't exist either? Who was Leif Erikson's Gilbert Stuart?
Ever go to the Natural Bridge in Virginia? There's initials carved in the rock there. Did Leif Erikson carve his initials anywhere?
the book is great big load of crap
And Gary Kasparov is an idiot, or a charlatan, or both?