Sounds like he pulled up early on mege and cut throttle to pull around verry tight and get a shot into you while you are pulling up and around in a bigger, energy conserving turn.
On a merge you can pull and immelman, which is a good merge but a verry common maneuver, so the other guy will be expecting this. But you can pull the immelman, which is halve a loop up then halve roll to get the plane upright again, "energy conserving" where you try to gain as much alt as possible and then inf the following fight try to use your better energy state. On the other hand you can cut throttle and pull the loop as tight as possible, including starting it early which will get you around quick and for a good shooting position if the other pilot does a standard energy conserving immelman.
Sounds like he did that.