You can't perfectly time your retirement to match the S&P 500 average. That's the reason your portfolio shifts to bonds as you get older. It's the reason everyone quotes 7 & 8% as reasonable long-term returns for your retirement portfolio.
Right, but if for each of the past 100 years you tracked people who put money in the stock market and held it for 45 years and averaged each of their returns you'd get a 7% YoY return after adjusting for inflation.
Some would end up lower but some would end up higher.
$2.1M is literally $100k compounded at 7% over 45 years.