he is talking about, in Diamond/Pearl, you breed Manaphy with Ditto and when the resulting Egg hatches you have a Fione. But Fione does not evolve into Manaphy.
I think that sucks, because you can only get 1 Manaphy, unless you have multiple copies of Ranger.
I wonder, if you were to trade the Manaphy egg, and then restart Ranger, would you get another Egg?