As for being a declarative statement, I think he means that this is a narration of a story. The imperative form used is something that Joseph (PBUH) said, not a command from God that may be subject to abrogation.
But stories in the Quran are not there for entertainment; they are to teach us values and laws. Jurists have disagreed whether
the laws of prior nations are laws for us too. Many scholars believe they are.
I agree to a point: We don't know what the laws of prior nations are! Because their scriptures have been edited by them. So, the only laws of prior nations that we can be certain were from God are the laws that God tells us about in the Quran. It is therefore incumbent upon Muslims to follow all the laws stated in the Quran, even if they were presented within stories of prior nations.
With that in mind, verse 12:101 was thought by those who saw in it a case of abrogation an approval of praying for death! If such was an approved value in prior nations, then it is for us too, since God tells us about it in the Quran. But since the hadeeth forbids praying for death, those scholars jumped to the abrogation explanation.
Their understanding is mistaken, IMHO, as I discussed in the OP.