I don't know about the conjecture re: Bowman, however I think you're hitting on something key, which is that very few GMs seem to be able to duplicate drafting success on their own. Scouting staff are crucial and it underscores how much of the actual management of your staff matters to running a good drafting/development program.
Benning may in fact be a great scout, but if he's a bad manager it won't matter much. It's also worth considering that scouting/development/drafting is a zero sum game, and as the league adapts methodologies that were once successful no longer are. It's highly likely that his methods were great in the early 2000s, and are simply mediocre now as organizations improve and access to information about players/video/stats etc is easier to get.