Their sources say "trade/migrate from another game". They're literally starters...
This generally has to do with the site as a whole rather than Pokémon itself, but as of this post, Sword/Shield and Brilliant Diamond/Shining Pearl are missing data for Gift Pokémon and Static Encounters. According to the site, starters from both of these games cannot be found where first obtained, their cover art legendaries are transfer exclusive, and Calyrex has never once been obtainable. Similarly, this site has yet to be updated with Scarlet/Violet's DLC Pokémon, so we're waiting on that too.
The reason behind this is likely to be that the site is run by one person, Pokemaster. He has gradually been less and less active as time has went on, and this is a sign of that. If you want an alternative, Bulbapedia has encounter data that's relatively complete.
