I know(I also used an Arceus when I did the calcs,lol.)I should clarify that the difference in power between the two is marginal, but you are technically speaking correct:
252 Atk Mega Lopunny High Jump Kick vs. 252 HP / 0 Def Arceus-Fighting: 187-222 (42.1 - 50%) -- 0.4% chance to 2HKO
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 252 HP / 0 Def Arceus-Fighting: 183-216 (41.2 - 48.6%) -- guaranteed 3HKO
Anyways I find funny when people say that M-Lopunny lack power against Bulky mons, but then say that Megagross does an stupid unwallable amount of damage(it has more coverage move tough,of course Megagross is overall better, but in terms of raw damage M-Lopunny is still stronger.)