AFAIK 5:4 is just the lowest ratio they've tested. Personally I use table sugar (1:1) and can sustain rates above 100g/h. Haven't hit the ceiling yet, don't really feel the need to explore where that is yet because exceeding the absorption rate comes with the risk of diarrhoea which is bad at any time but especially when you're in the middle of a training session and who knows where the nearest toilet is.

Gut training is consuming large amounts of carbohydrate (preferably in the same form you intend to use when racing), yes.

Yes but the science is actually achieving that and finding the limits. It used to be thought that 60g carbs/hour was the limit, then 100g, now it’s thought to be 120g.

It’s also about the methods of achieving that under stress without spewing it all back up. Ironman athletes would stuff their faces on the bike under the assumption that this volume of carb absorption wasn’t possible while running.

Some of the challenge in research will come from competitors not wanting to publish results to maintain an edge. It is mitigated by the visual of the race by (you can see athletes pounding carbs), as well as the nutrition companies wanting to sell more product. This will cause them to publish some information to convince us amateurs to quadruple our purchase volume ;-)