Why do you think he felt the need to add it if it was already there?
Rom 3:
28
For we consider that a person is justified by faith apart from works of the law.
Martin Luther added ‘alone’ after faith, and interpreted works of the (Mosaic) law (such as diet, circumcision etc) as meaning ‘good works’.
Changes the entire meaning; no wonder some generations appear to have been seriously misled.