Get film and then study it. Physics is physics. It is relatively easy for a higher e fighter to immel at the merge and be on his opponenet's six if his oppoenent straight lines it. More precisely, the high e fighter is doing a half cuban 8. Immel over the top until the nose is below the horizon, role upright and accelerate in an unloaded dive toward the opponent.
When you see the film you'll see one,or a combination of differences that will make it clear how it was done. First, you may find that he actually had an altitude advantage which he converted to speed by diving down to your altitude, maybe even before you saw him. Second, he probably was set up for a lead turn on the merge giving him even less degrees-to-go to get his nose back on. These claims of a "magical" reversal, over-modeled airplanes, even "hacking" have been around for years and I've never once seen a film that shows anything but that which can be explained by one or both of the above. Pilots need to understand that they will probably not remember everything exactly as it actually occured, that's why you should always film and review the film if something happens that you don't understand. If people would only do that I think Ch 200 would be a lot quieter.
One of my favorite instances of this was when I did this in a Hurricane against a Tempest. Ch 200 was a tirade of indignation and accusations from the Tempest pilot. He was being chased directly toward me, I dove down accelerating to about 400mph, did a lead turn and dove right in on his six. He claimed that he was going 450 (film showed him at about 300). He claimed I climbed up to him (he only saw me after I had dove down below him and started pulling up for a vertical lead turn). I was almost pure vertical at the merge which means I only had about 100-120 degrees to turn. He claimed I "warped myself" onto his six (film showed nothing but a half cuban 8). I used to have this film but lost in when I had to do a complete reinstallation of windows on my PC.