Great question and it sounds like you have good knowledge around this subject. Yes endurance athletes with training do become better at using fat as fuel source regardless of dietary intake. Carbohydrates are actually more efficient at yielding energy (ATP) as you require less oxygen to produce the energy compared to fat, however you do yield more ATP from fat but this requires more oxygen for this process. So depending on the intensity of your training (and the resulting oxygen demand required to carry out this training will influence the fuel demand, carbohydrates vs fat). You can influence which fuel is predominantly utilised depending on a number of factors including training intensity, duration, but also training level, age and gender and also nutrition intake. Here is a nice review https://www.sciencedirect.com/science/article/abs/pii/S1751499111000060 explaining fuel utilisation during rest and activity.
The idea is that as you train for endurance and use train low nutritional strategies (low carbohydrate intake strategically planned around low intensity training) you can further enhance fat adaptation (without the need to completely restrict carbohydrate from the diet). If you completely omit carbohydrate from the diet, then this has shown you down regulate your muscles ability to use carbohydrate as a fuel source, so essentially you lose your top gear, power output will be reduced. Which may not be a problem for prolonged lower intensity events/races, however I see no reason as to why you would not want to utilise both fuel sources and become what is deemed as fuel efficient. You can then tap into both sources this way, tapping into carbohydrate when you power uphill, the ascent home or hard scramble/effort/climb during mountaineering but train your body to use your most abundant fuel source within the body, which is fat during lower intensity prolonged effort.
Nutrition guidance for ultra endurance is changing suggesting fuel for the effort required, periodising nutrition intake (mainly carbohydrate) relative to the training demand and adaptation sought, as well as the profile of the main event.
I hope that has answered your question? It’s quite an in-depth and extensive topic and if you would like more of the spefici science I would be happy to post up links to this further research to read.
Rebecca Uphill Athlete Dietitian