Actually, the 2 Pokémon will breed even if the man says this. It happened to my Samurott with my Ditto and I had an egg. Though, you have a 20 % of chance at each 256 steps to get an egg, increased to 40 % if you have an Oval Charm. This actually happens because your 2 Pokémon has the same ID number, but are not of the same species.
Actually, Fizz said it all, I just added some extra informations. If you transfer an Adamant Gyarados got from another ID number, simply another game, you can increase the chances of getting your egge.
Source, + Experience, Fizz and my brain.