All muscles are capable of metabolizing fat. In cycling (and other endurance sports), one of the adaptations observed in top athletes is that their muscles become highly efficient at metabolizing fat during medium-intensity exercise. A professional endurance athlete will metabolize about 70% fat, 30% carbs for the majority of a multi-hour event. This preserves their muscle glycogen for the high-intensity bits where they need to push 400+ watts for 20-30 minutes up a final climb, or do a 1200w sprint to the finish line. When the intensity level exceeds a threshold, the muscle will begin switching to nearly 100% glycogen. Once that glycogen is depleted, muscles lose their top-end peak power output.

Sedentary overweight people tend to become very inefficient at metabolizing fat. At anything higher than a slow walking pace, for example, they will begin the cutover to glycogen and turn down fat metabolism.

I believe the press release is saying that the soleus muscle is unique in that it does not have a readily accessible store of glycogen. So even in sedentary people that are normally extremely inefficient at metabolizing fat, exercising the soleus will force their body to metabolize blood glucose and fat. Normally it takes months or even years of slow and steady exercise to make a sedentary overweight person effectively metabolize fat while exercising at an intensity high enough to trigger serious metabolic improvements. So if true, the soleus muscle would be a magic shortcut to this process.