I also probably wouldn't have adjusted my speed. Passing at a normal pace, probably just lifting a hand to say hi rather than smashing it past and verbally doing it. Possibly thought, as you say, that you were overtaking at speed and then slowing back down just to one up them (hence why they did it back, to prove a point: I can do that too asshole). I think minding my own business and waving is the best general policy
I also probably wouldn't have adjusted my speed. Passing at a normal pace, probably just lifting a hand to say hi rather than smashing it past and verbally doing it. Possibly thought, as you say, that you were overtaking at speed and then slowing back down just to one up them (hence why they did it back, to prove a point: I can do that too asshole). I think minding my own business and waving is the best general policy