It is impossible to adequately describe how DACT between friendly nations is done. Gambit made a very good effort. The public commonly thinks of them as all-out brawls, like boxing. Two heavyweights go at it, and only one remains standing. It is NEVER like that in DACT.
In my years of ACT/DACT, every single mission had what are called "learning objectives", and one or both sides act a role, so as to maximize training. When the USAF aggressors came to support a unit, everyone outside of the environment thinks "The Aggressors will go in and fight as hard as they can; may the best man win." It doesn't work that way. I (in an instructor role) may take a new wingman, and make use of a single Aggressor, placing him in what are called perch setups, canned setups designed to train the new wingman. I may handcuff the Aggressor further, telling him he is limited to AIM-9P equivalent. I'll have him start behind on some setups, and I'll have him start in front, so that I may teach the new wingman the art of good COMM and maneuvering with a partner in both offensive and defensive setups.
This can get expanded into a 4 V 4. I will ask the Aggressors on the first engagement to execute a lead-trail formation, with the leaders dragging out, or some sort of difficult sorting problem for our flight. On the next, I may say "We are all AIM-9P and guns" meaning no BVR, no front-aspect shots. Everything is crafted to maximize training. I "died" hundreds if not thousands of times in training. That's how you get better.
In this sort of situation as described, the F-15's may be acting in an Aggressor role. They may have been intentionally limited. Or, it is entirely possible they got waxed. That doesn't mean that the next day, they won't crush the opponent. That's how it goes from day to day.
One other thing people rarely consider is the psychological factor of watching your friends and flight leaders go down in flames. It is easy to say "8 MiG-21's can kill 2 F-15's", but I will guarantee when your friends are blowing up all around you, and there are shouts and frantic calls on the radio, the best plan in the world will flush right down the toilet.