Those calculations are a bit flawed in that the target (Landorus) have much, much higher Def than Sp Def. If we look at just the moves and Aggrons base Atk/Sp Atk, we get the following calcs:
0 Atk Mega Aggron Ice Punch vs. 0 HP / 0 Def Arceus: 62-74 (16.2 - 19.4%) -- possible 6HKO
0 SpA Mega Aggron Blizzard vs. 0 HP / 0 SpD Arceus: 45-54 (11.8 - 14.1%) -- possible 8HKO
Eliminating all other factors, Ice Punch is far more damaging, and will be still more powerful with EV investments, etc.