According to Guerrilla the Burning Shores is more graphically and technically advanced than the base game to the extent that the PS4 can't effectively handle it. That's why it's a PS5 exclusive, sadly for me and all the other PS4 users.
I can understand the need to leave behind older generations, even if it is frustrating. And while it is a DLC, studios have to keep moving and advancing to keep up with competition.