It's true. But as a fighter cardio for you isn't a steady 20-30 minute run. When we talk about cardio what we are really looking for in a fighter is an increased anaerobic threshold.
Try more interval, training, more 400m sprints, more fartlek training, and mix in some circuituos type lifting with your regular lifting regimen. Remember quality is more important than quantity.
Short, high intensity intervals require the ability to quickly restore immediate energy supplies so that another burst can be performed. Furthermore, in instances when sustained, high energy is required, high levels of lactic acid produce an acidic environment that will shut down muscle contraction. The ability to quickly recharge the immediate energy stores within the muscle, and to buffer the acid in the system (i.e. the Hydrogen), are NOT improved significantly by steadily jogging a few miles! If you want to create these very important adaptations remember