I think a lot of people was confused about the new pokemon even i think it already clear as they said in the direct and also on the DLC website, first of all, your "basic/standard" sword and shield when you bought it from the store only have a data for 400 Pokemons along with about 30+ pokemons that you can bring it in with home and some event(like Kanto, Alola starters)and then when the DLC direct was showed, your "basic/standard" got a data update, now you can meet, catch and use galarain slowpoke
See what it mean? As they said, when the DLC keep coming out, your data in your "basic/standard" sword and shield game keep getting bigger and bigger as the data for the new Pokemon get into your software, it a free update, Masuda said it in the direct, if you don't buy DLC, bang! You will get a free data update of return Pokemon in DLC along with the data of the new one, otherwise, when you ran into some people who use Zarude or Kubfu in the raid, your game will not able to tell what the freak is that thing suppose to be and bang, you game crash but since all of the data for those Pokemon are free update, once you update it, if you see them in raid, online camp or online battle, your game work fine, you can even begging for people to trade you those new Pokemon and you will able to use it just fine without buying the DLC (by the way, when i said game data, i mean the model, animation, move data and all)
Zarude is the same, it might be not get his own event to catch like the past games but at that time, the game will update your basic data and you can get your own Zarude just fine, even with DLC or not
By the way, on the ticket code website, it said you can use code for Shiny celebi and Zarude even you don't buy DLC so it already clear from the start.