Here God deciding not to do the punishment he had planned is spoken of is if he changed his mind. Alternate translation: "God changed his thinking about the punishment that he had said he would do to them" or "God decided not to punish them as he had said he would" (See: [[rc://en/ta/man/translate/figs-metaphor]])
What God was to do can be made explicit. Alternate translation: "he did not punish them" or "he did not destroy them" (See: [[rc://en/ta/man/translate/figs-explicit]])