In those verses the 10C were part of the law being referred to, so don't understand your objection.
Because the poster in question has an agenda in which only the Decalogue are to be regarded as God's commandments, for the purpose of arguing that their particular sect--the Seventh Day Adventists--are God's true Church; as only those who observe the Decalogue, especially the keeping of the Sabbath, are truly abiding in the truth of God. I'm simply pointing out that they are inserting their own sectarian doctrine into the text.
Yes, the Decalogue is part of the Torah, but it's not the only part of God's Torah. And not only that, the Torah isn't the only Law God has.
Christ commands "Do not resist an evil person, if someone strikes you on the one cheek, turn and offer the other as well." That's not in the 10 Commandments, it's not in the Torah at all. But it is a commandment, it is God's Law.
For Lutherans the idea of Law means simply this: All which God has commanded is Law. So when we read that no one is justified under the Law, yes it means that no one is justified by observing the 613 mitzvot of the Torah--but it is also part of Paul's repeated refrain throughout his writings that no one is righteous by works, by effort, there is no earning a place with God by trying to be holy enough, righteous enough, good enough since all have fallen short, all sin, and thus our justification, our holiness, our place with God comes from outside of ourselves, as pure gift--as
grace--by what God Himself has done for us in Jesus Christ, freely justifying us, and appropriating to us all the things of Christ as pure, unmerited gift.
The Law brings knowledge of sin because the Law condemns sin; so that when I hear the commandment my conscience is pricked as I find myself a sinner under the Law. And so the Law brings death. And no one can be justified by the Law, no one can be holy under the Law; because before the Law all stand condemned as sinners, dead in their trespasses. Both the Jew and the Gentile.
-CryptoLutheran