The author mentioned "the table should now have no problems for the next 6 to 9 Billion years"

if the usage of the table was to remain the same would the following not be true?

max value for signed int = ((2 ^ 32) / 2) - 1 = 2147483647

rows per year = (rowcount / monthsInOperation) * 12 = (2147483647 / 8) * 12 = 3221225471

-> rowcount chosen as max value of signed int cause it was the datatype to overflow and monthsInOperation from artical

max value of signed bigint = ((2 ^ 64) / 2) - 1 = 9.22337E+18

number of years = max value of signed bigint / rows per year = 9.22337E+18 / 3221225471 = 2863311532

a mere 2.8 Billion years (yea I know someone else will have my job by then), somewhat less than the 6 to 9 that the author claimed

If we are talking about understanding the demands on our data types and choosing them wisely it seems such calculations should be done. I can imagine the designer thinking int would be more than enough, don't do the same with again by not taking into account such simple calculations.