It’s not self employment income to the surviving spouse. Here are two sources for you:
Publication 559 states that “for self-employment tax purposes only, the decedent’s self-employment income will include the decedent’s distributive share of a partnership’s income or loss through the end of the month in which death occurred.”
Rev. Rul. 59-162: held that commissions on insurance policies that were paid to a surviving spouse were not self-employment earnings to the surviving spouse, even though they would have been self-employment earnings had the deceased spouse received them during his life. This Revenue Ruling is very short and clearly sets forth the rule for you.