what he is saying is that they aren't actually diving.. that's an artistic representation.
so, he didn't actaulyl dive that many times, most likely... and the error rate.. simply look at his fielding percentage. they make errors and those errors will split up in a proportionate way relative to your League Totals and modifiers. maybe even some other miscellaneous settings.
whether it's a blanket error % or it actually differentiates based on difficulty of the play somehow, i have no idea. i think it's just a blanket % relative to the player. not outlandish to think the random seed the batter rolled is important too
a harder hit ball has to be more difficult right? plus, it already exists in game to some extent with how balls in play are dealt with - possibly.