It says it created all Pokémon, but let's consider this:
Arceus cannot learn every attack, unlike Mew. And while it can change into any type (excluding ???-type), that makes it have that many more weaknesses. Also, while it has the highest BST, it does NOT have the highest base stats. While they're all 120, many Pokémon out-base him. Marowak for attack, Blissey for HP, Ninjask for speed, etc. So whether Arceus is the strongest is widely debatable.