How much of that "has shown to be" is just confirmation bias, though? Seems to me as likely to be a dampener on "finding talent" when you have to compete with FAANG (or MANGA, or whatever).
Additionally, the talent pool skews much younger in hot markets who are on average much more likely to be willing to take a risk and work really hard to make a startup take off.
Obligatory disclaimer that I'm not saying non-hot markets are not filled with great devs and that older people cannot make a startup take off, just the odds are better in one vs. the other.