Well I guess the idea is that all the "best" engineers would have already made millions by the time they hit 50 and so therefore those who are left are...
People who think this way are blinded by their own worldview. A large number of the best engineers aren't in the business to become wealthy, and they don't leave the industry if they become wealthy.
The ones who are in it just for the money rarely become the best engineers.
Perhaps in the hyper inflated SV economy, but in the rest of the world you don't get to retire until you've reached the normal retirement age (minus a few years perhaps), and that makes it quite natural to see newly employed software engineers who are in their mid or even late 50s. I've worked with plenty of them.
I must admit though that I've only seen one newly hired in their 60s, but probably the very few that find themselves involuntarily out of a job at that age can afford to and choose to retire a little bit earlier than planned.
Once you get into late 50s/60s, I know quite a few people in tech at that point who have had reasonably well-paying jobs (though certainly not top-level FAANG salaries) who have presumably managed their finances well who are pretty much ready to wind down at that point--whether forced or otherwise. They mostly haven't classically retired but they're working part-time on often IT-adjacent types of projects.
The ones who are in it just for the money rarely become the best engineers.