He is right that in hermeneutics we have something call descriptive texts and prescriptive texts.
But there is something else we call selective citations and the analogy of Faith where we harmonize Scripture with Scripture systematically.
Let us grant for a moment that Deut and Exodus can be considered to be something other than about polygyny. Even if we grant that this speaker/teacher failed to address the Levirate law. In the 7 minute mark he stated that to take any second lady to oneself is to take a sister and thus this forbids the practice all together. If that is so then the law of non-contradiction would be violated in two places.
1. The levirate law commanded it in some cases.
2. He is guilty of not dealing with the 2nd Sam text in its fullness and he sets laws against laws under the same Mosaic Law Code.
In 2nd Sam we have a statement whereby a holy, Righteous, and Perfect God told David he could have been given more ladies had he simply asked. Furthermore, we have a statement where God gave him the multiple ladies to begin with. Oddly though the teacher here did not mention that part of the verse.
Thus, even if we were to grant him the position he takes on Deut and Ex, which I would not agree with, we still have other descriptive texts and even with the Levirate law a prescriptive text where God approves and even called some to that lifestyle.
What this writer does not realize that if he proves his case then he has inserted a contradiction into the Word and left us with a book that is questionable then as to its supernaturtal inspiration.
Thus, if his ideas lead to an undermining of that then it is best to back up and seek a better systematic answer that sees the harmony of all texts with one another.
This leads us to the third point. There is a law in hermeneutics where we interpret the clear over the unclear verses. If there is a dispute over how those two laws in Ex. and Deut. can be interpreted then we go to areas where the text is generally not disputed. Few argue against the levirate law and few will argue that in 2 Sam we see the text reading that God actually gave David multiple ladies and was not charged with adultery. Furthermore, Lev. 18 lists every type of sexual sin that can be imagined and not once is polygyny mentioned in the clearest and strongest passage that denounces sexual sin.
Thus those clear texts govern how we would approach an unclear text wherever find a disputed text.
That is how I approach such matters from the way I have been discipled.
Dr. Allen