Sunday was the first time I've done this. And it worked well logistically, but no idea whether it made a difference in the result. My assumption is 98% of your performance is due to training & in-race pacing strategy, 2% due to fuel/hydration.

For the most part, I would agree.  The major thing folks try to avoid in marathons is, obviously, running out of glycogen.  The dreaded "wall."  That can be avoided through fairly simple precautions.

First, the majority of folks aren't running at an intensity at which running out of glycogen is really an issue.  Think about this.  You could go stroll a marathon in 8+ hours without taking in anything and you wouldn't run any danger of running out of glycogen.  At that level of intensity, the vast majority of your calories will come from fat.  Recreational runners simply are not running at the same level of intensity as elite runners.  No one can, for example, run for 3 hours at the same level of intensity as they run for 2 hours.  Thus, contrary to what a lot of folks think, the slower you are, the less need you have for fuel during the race because you have an essentially unlimited supply of fat.

Second, most people are aware of the need to begin a good carb load a couple of days before a marathon and they do so.  Thus, they start the race with glycogen levels topped off.

Last, most folks will hit the aid stations or take some kind of gel with them so they'll replenish a few carbs along the way.

Almost invariably, if someone experiences problems in the last few miles of a marathon, it's because they were dumb enough to believe they could run a McMillan equivalent time on 40 miles a week or because they went out way too hard in the early miles.