The Indigo Disk, the DLC for Pokémon Scarlet and Violet, has its own Pokédex, since it takes place in a new area: Blueberry Academy. There are more than 200 Pokémon in the Blueberry Pokédex ...

To unlock the Iron Boulder encounter, you'll have to fill up your Pokedex to 200 entries, meaning you either need to upgrade the Teratium in the League Club Room or have access to Pokemon Home.