The amazing thing here is there is no reference to scripture in these verses. We have the law of the Lord, the precepts of the Lord, the statutes of the Lord, the commands of the Lord, etc. So he starts with the assumption that these phrases are talking only about scripture. He even does this when his stated goal is to prove the sufficiency of scripture.7 The law of the LORD is perfect,reviving the soul.The statutes of the LORD are trustworthy,making wise the simple.
8 The precepts of the LORD are right,giving joy to the heart.The commands of the LORD are radiant,giving light to the eyes.
9 The fear of the LORD is pure,enduring forever.The ordinances of the LORD are sureand altogether righteous.
This is how deeply embedded assumptions about Sola Scriptura are. A bright guy like MacArthur cannot even address them when he tries. He sets himself the task of proving scripture is sufficient and proves only the Word of God is sufficient. He can't even address the possibility that the Word of God might be bigger than just scripture. So much so that he declares a text that does not mention scripture at all to be the clearest, most concise statement of a principle about scripture.
The amazing part is that I know nobody is going to see the hole on his logic. I do because I have been thinking about such questions for a long time now. But 10 years ago I would have found his argument very convincing. I would never have noticed his huge question beg. That is because I shared the same reformed tradition. I had the same embedded assumptions. So I would have listened and has my ideas reinforced just like most of his listeners did. I would have said that my intelligence and John MacArthur's intelligence would allow me to be sure no error was made.