Lego's argument always struck me as absurd. Imagine Ford doing the same thing: "Please refer to our vehicles as 'Ford vehicles' or 'cars', and not 'Fords'. By doing so..."
The church does have extrabiblical volumes of scripture, which also testify of Christ and go into more depth, but every doctrine of the church is in harmony with the Old and New Testaments. Members believe it is only by faith in Christ and by good works that they can be saved.