I understand WHY the law existed. It does not change the rebuttal to the original assertion that God never commanded polygamy. The fact is simply this. God did give a commandment that could quite possibly result in polygamy. Further, there are OT laws about how plural wives and even concubines are treated.
Deu 25:5-6 is a flat rebuttal of the assertion of the author.
And which one would that be?