Mitzvah after death

I saw it in Asya’s (Ez) pamphlet [pamphlets on medical and halakhic matters, (if I remember correctly the rabbi himself wrote an article in one of the issues on statistics in halakhic matters)]
A story about a guy who was diagnosed with cancer and since the treatments were supposed to affect his ability to have children, his sperm was taken into a test tube and frozen in a refrigerator. In fact, the guy died, and three years later, he had twins, a boy and a girl, from that same sperm.
I asked whether the above-mentioned person fulfilled the mitzvah of “be fruitful and multiply” or whether, since it was after his death and the dead are exempt from the mitzvah, it does not exist (and is this similar to assuming a fetus and having children born to him after his death, or does the entire mitzvah here begin after his death, when the sperm was inserted into the woman’s body two years after his death?)
With great respect
Israel