Simulators are used quite regularly to practice combat tactics! In fact, that's one of the intended uses of the Joint Simulation Environment that the article discusses.

DCS's missile simulation models are so far from the real thing that DCS is what simulation engineers refer to as "negative training". That is, if you train to fight using DCS, you're training to fight within DCS, not training in a way that's going to carry over to the real world. DCS is fine for what it is -- a simulation that reflects as much of the pilot/vehicle interface of various aircraft as is possible. But when it comes to sensors and weapons, it's very far from reality.

The Alpha Dogfight trials were a PR stunt but they were also a part of a larger program to see how AI technology can assist human pilots. Alpha Dogfight is akin to things like Deep Blue/Alpha Go in that it has opened up a whole domain to further research and deployment of AI systems to assist humans.