Flat out lie. Based on false assumptions. They assume that every nuclear blast results in a firestorm like Hiroshima which was almost entirely wood built and had not had rain in weeks. Nagasaki had no firestorm.
Carl Sagan lied about nuclear winter. That worked so well Carl Sagan and friends made up data for global warming. It is all a lie to promote their agenda.
Scientists with an agenda? I find that hard to believe.