If I understand correctly, both the staging database and the production database share the same volume. Thus, production data was gone as well after deleting the volume.
1st hint - the API call only contains one volume:
curl -X POST https://backboard.railway.app/graphql/v2 \
-H "Authorization: Bearer [token]" \
-d '{"query":"mutation { volumeDelete(volumeId: \"3d2c42fb-...\") }"}'
2nd hint - this gem from the tweet:> No "this volume contains production data, are you sure?"