Sn is the future value. Set the variable r = (1+i)/(1+g), where g is the growth rate and i is the interest rate. Set the variable a = p(1+g)^(n-1), where p is the first payment. The payments in Sn are listed in reverse chronological order. The future value of the first payment in Sn is ar^(n-1) = a(1+i)^(n-1)/(1+g)^(n-1) = p(1+i)^(n-1). The future value of the last payment is a = p(1+g)^(n-1).
There is something in the spreadsheet I found puzzling. The entry FV is the inflation-adjusted value of PV. It is the amount required in the future to equal the purchasing power of PV. These are not the standard meanings of FV (Future Value) and PV (Present Value). The standard definitions of FV and PV involve the time value of money, which is measured by an interest rate, not an inflation rate. FV and PV in the spreadsheet are looking at the purchasing power of money, not the time value of money.
So I find it puzzling that when you accumulate the value of the annuity payment, you do use the time value of money, but then you compare it to FV which uses the purchasing power of money. To me that is inconsistent and so I would like to know what you have in mind when you do that.