The owner is DBO
This is the reason. Only the owner of a data, will have the option to delete it. So in this case, any user with SYSADMIN server role assigned to them can be used to delete the Feature class.
If I look at the privileges for the feature classes using ArcCatalog, it says that DBEditors has Select, Insert, Update, and Delete privileges.
These Select, Insert, Update, and Delete are pointing to the permission to edit data inside the feature class. You can select(view) an existing, Insert new records, Update existing records and Delete a record from that feature class, with these privileges granted.