The pro cyclist and other athletes are better at burning fat because of their constant training and energy demands of their sport. Not directly because of any particular diet. We'll increase our fat burning capability just from riding more often, faster and longer.
Bonking is really something that noobs do IMO. And many probably don't bonk in the sense that they can't ride one pedal more. They just can't keep up. After any of us get a little experience and fitter at riding we naturally learn we can't go all out all the time and figure out where to conserve and expend energy. Fat does fuel much of a ride. When we learn not to exceed that rate of fat burning during certain easy parts of the ride, we can replenish the glycogen that we might have just expended on a hard effort or a sprint to get to the front of the pack or just to keep a higher than normal speed.
At high efforts glycogen will be depleted faster than it can be replaced. Carb's help to replace glycogen quickly but never as quickly as one can burn it when going hard. As well the stomach/gut won't absorb carb's fast enough to ever replete lost muscle glycogen on a ride. So if one knows they are going to be going into very high efforts at various points along the ride, it'll probably be best to just regularly consume carb's from the start just to make certain the glycogen is as topped off as you can get it.
Though consuming those sugars in excess will probably just make many feel bad or queasy in the stomach and that bad feeling will impact performance. My stomach doesn't like more than about 200 - 220 Calories per hour. Maybe 250 if I push really hard. Typically I'm consuming 120 to 180 Calories in 50 minutes. (all my Calories are in my bottles. 24 to 25 oz bottle last me 50 minutes). Other cyclist will vary on how much sugar they can tolerate. There are studies that show how much the average person and average athlete can digest/absorb while cycling. Everyone should find their own limits when they don't have a professional trainer, coach, dietitian or other monitoring them.
Sometimes in the winter, I'll go out with just water. However I don't know that that has done anything for improving my fat burning. It was once touted that some pro's thought it helped, but turns out maybe it was the EPO or other stuff they were using.
Despite the strong feelings some have about increasing levels of their own bodies fat burning. It's not going to be an overall strategy for anything. Nor is diet type going to play more into increasing ones fat burning than simply getting out and riding more.