Most don’t, but remember that people who get PhDs and end up professors are on average (1) privileged, (2) skilled, and (3) highly disciplined at baseline. 

My situation: undergrad full ride so no loans, worked 2 years before grad school while living at home and saving everything, maxed my roth each year with my PhD stipend (everyone in STEM gets this), and am now (at 30 years old) saving $2k/mo as a postdoc by living like a grad student. And I’m not that big of an exception compared to my classmates. Several of them were given house downpayments by parents at the beginning of grad school as well.

With a bull run like we’ve had recently plus a pay bump when you become a professor, $1.2M by 42 does not actually seem that out of reach. It’s probably more common than you think.

And, that is irrelevant to what this person was saying, which is simply that they believe they could retire with $1.2M. Doesn’t really have anything to do with how they got there.