Yes, it's true, but not all of them. For example, you can't get event Pokemon.(Like Jirachi) You do need the national pokedex. You don't need to get all the legendarys, like Virizion and Kyurem, but they are good to keep. Now, you can find Gen I-IV in Pokemon Swarms/regular grass/water. For the legendaries/others, you have to Poke-Transfer.