Path of least resistance I think is the majority of it. I don't know that I would say it is preferred just because it would use it first if it had both. Fat has to be broken down to be used, glucose can be used more or less directly, it makes much more sense to quickly use carbs while they are 'free' than it does to go through the process of turning them to fat, then breaking them back out again to burn. The body is just being efficient and using it in its raw form and not storing it.
Another point is: do they prefer it? When you are fat adapted, and doing some aerobic work (link to FASTER study which has a lot of info) you can see below that people that are fat adapted have much higher fat oxidation levels.
Even the chart on the left (HCD=High Carb Diet) shows ~50% of energy coming from fat during the duration of a 3 hour endurance workout, if carbs were truly the preferred source then would we not expect near 100% carb oxidation during the first 30 minutes to 1 hour and then all fat after that? The low carb folks use ~90% fat during their 3 hour workout, that is definitely not a glucose preference.
They also did muscle biopsies after the workout and showed similar levels of glycogen remaining, so the LCD group didn't burn it because the had to, they did because they have become adapted to using fat efficiently as a fuel source.
When you look at anaerobic workouts you see that carbohydrates are used almost 100% which is why people tire out quickly. This goes back to the earlier discussion about efficiency, during a sprint there isn't enough time to convert fat fast enough, so it uses the glucose it already has stored in the muscle.
Edit to add one piece to this. Glycogen is stored in the liver and muscle, and is 'preferred' especially for high intensity activity, but if you are keto and don't eat carbs then where does that glycogen come from? As you probably know glucose is a product of fat metabolism, so as we burn fat we slice off some fatty acids and turn the rest to glycogen to store in the muscle. So while technically the material you are using to make that muscle work in high intensity exercise is a carb, it came from metabolizing fat earlier in the day.