Even with bad market timing, you still have pretty good odds if you are genuinely dedicated to the task. Keep in mind, to succeed in ~20 years in almost all market conditions you would need to save 50-60% of your before-tax income.
His math is a little optimistic when we are talking about "almost all market conditions" and once you start factoring in things that might go wrong, like a couple years of unemployment stretched over a 20 year career.
MMM ignores a ton of statistically common problems (like 4 bouts of unemployment over 20 years).
"[A] couple years of unemployment stretched over a 20 year career" seems to be an extreme outlier for the tech field.
I worked through the 2000 and 2008 crashes and I can't think of anyone I worked with in tech who was involuntarily unemployed for more than a few weeks at a time and certainly not for 10% of the total time. (Voluntarily for personal reasons like they decided to stay home with a kid, sure.)
Yes, years or 10%+ of unemployment would derail retirement savings. A few weeks here and there doesn't.
https://www.mrmoneymustache.com/2012/01/13/the-shockingly-si...
Under his assumptions, to retire in 20 years takes a savings rate of around 44%.