I can only speak to my personal experience. I’m not an athlete but I lift, bike and so on. On empty stomach. I don’t eat before training. I could never do that on carb diet.
I follow TFTNA (Training for the New Alpinism) and Uphill Athlete (Steve/Scott's new book) training regiments that Parent mentions.
They do talk about retooling your body to use fat reserves more than carbs. While I'm sure the evidence shows that keto diet might make this switch faster and easier, someone on a non-keto diet (carb based) can also do this.
My other very favorite resource is Shawn Bearden's Science of Ultra podcast, which is where I learned of the research paper I linked in my first post. Here's the one on fat adaptation: https://www.scienceofultra.com/podcasts/19 . He has lots of pretty hardcore exercise scientists and researchers on. It's one of my favorite things to listen to while grinding up a hill. :)
Have you tried? I always run before eating (not as part of a regimen, it's just convenient), as I expect many do. And my diet is closer to inverse-keto than keto.
I think that your comment falls into the noise category and is undesirable because of that. If you had explained personal experiences to show how you reached your conclusion, It think you would get a better reaction. If you also showed how other possibilities could be rules out, then I think people would find it very interesting and you would get many up votes.
Also, looking at comments, often I see only upvote and no downvote button. Why is that?
I'm not saying my personal experience should be considered universal, that'd be foolish. Just recounting that.
But it makes sense given how metabolizing fat and metabolizing sugar works. Insulin response, etc.