This is a bad move by Nintendo.
To their credit, when you're in SwSh, the link to buy the DLC that's in the pause menu takes you to the correct version in the eShop, and the versions are labeled "Sword Expansion Pass" and "Shield Expansion Pass". When they announced the DLC, I remember there being some confusion over this issue, but I think they clarified at some point. So, I assume they knew this would be an issue.