If we read enough in Luther, we discover that he saw the fulfillment of the Law through Christ’s life, ministry, death, and resurrection. Thus, the 10 commandments as formalized at Mt. Sinai are not for Christians. Rather, the re-creation in Christ means that we are back to creation and the relationships formed there. While the 10 commandments give a summary of the moral law and the creation relationship, it is their fuller understanding as revealed in Christ that carries the day. Thus, Luther looked at the four estates of creation as the basis for life. From his perspective, the “authority” given in creation begins with husband wife, then to children, and ultimately to all who are in authority.
If we take the 10 commandments as legalistic absolutes, then yeah, we will have a problem with how Luther works with the commandments (consider even the 3rd commandment and his explanation which is even more). But if we understand the greater fulfillment in Christ, then his explanations not only make sense, but reveal a much broader understanding of what it means to be God’s “creature” (1st Article of the Creed), “who has been redeemed” (2nd Article of the Creed), and who ”calls, gathers, and sanctifies” (3rd Article of the Creed). That is why Luther could expand on the commandments and yet be consistent with the Gospel as predominant. Also, this matches Paul’s puzzling statement in Galatians 5:7.
In other words, Luther understands everything Christocentrically, not legalistically. This also separates us from both the RCC/EO and the Reformed (in the large sense of the word). Read his Large Catechism to catch the full nature of his approach, especially in regard to the 1st Commandment and the 1st Article of the Creed.