The Ten Commandments as the summary of the moral law

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Puritan Board Senior
This is probably a very elementary question, but how do we know that the Decalogue is truly the summary of the moral law, rather than simply a statement of ten different laws?

The Westminster Larger Catechism, question 98, reads:
Q. 98. Where is the moral law summarily comprehended?

A. The moral law is summarily comprehended in the ten commandments, which were delivered by the voice of God upon Mount Sinai, and written by him in two tables of stone;[420] and are recorded in the twentieth chapter of Exodus. The four first commandments containing our duty to God, and the other six our duty to man.[421]​

The prooftexts for [420] are Deut. 10:4 and Ex. 34:1-4, but to my knowledge neither of these explain that the Ten Commandments contain a summary of the moral law. The prooftext for [421] (Matt. 22:37-40) states that "all the law and the prophets" are comprehended in the two greatest laws, love of God and of neighbor, but I am unclear as to how that would imply that the Ten Commandments summarize those two laws.

When the Catechism expounds on the various duties and sins implicit in each commandment, other Scriptures containing moral imperatives without reference to the Decalogue are cited. This presupposes that the moral law is summarily comprehended in the Decalogue; therefore I was interested in understanding why Reformed theologians have treated the Decalogue as such.
I believe that both Christ and the rich young man both presuppose this in their dialogue in Matthew 19:16-19:

16 And behold, a man came up to him, saying, “Teacher, what good deed must I do to have eternal life?” 17 And he said to him, “Why do you ask me about what is good? There is only one who is good. If you would enter life, keep the commandments.” 18 He said to him, “Which ones?” And Jesus said, “You shall not murder, You shall not commit adultery, You shall not steal, You shall not bear false witness, 19 Honor your father and mother, and, You shall love your neighbor as yourself.” 20 The young man said to him, “All these I have kept. What do I still lack?”

Christ rhetorically asks about "what is good" (v. 17) and I believe he is referring to the whole of what the moral law defines.
Romans 13:9-10 For the commandments, "You shall not commit adultery," "You shall not murder," "You shall not steal," "You shall not bear false witness," "You shall not covet," and if there is any other commandment, are all summed up in this saying, namely, "You shall love your neighbor as yourself." Love does no harm to a neighbor; therefore love is the fulfillment of the law.
I think that Scripture itself provides us with the concept of "summary", Ex.34:28:
Moses was there with the LORD forty days and forty nights without eating bread or drinking water. And he wrote on the tablets the words of the covenant—the Ten Commandments.​
What was inscribed on the Tablets? The Ten. Why those Ten? Were they random? Or are they the cornerstone on which the rest of the legislation rests?

Jesus' testimony concerning the law seems to sum up the two "branches" of that law, love to God, and love our neighbor (quoting from elsewhere in the lengthier legislation), clearly with application to the foundational 10.
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