What makes you think this is true and not just a post-hoc rationalization? Let me put it this way: offer that salary worldwide and you will still have difficulty finding top-tier talent, as many remote-first companies are finding out the hard way. Having been a hiring manager in such a position before, I will tell you that you are still competing against every other remote tech company to find and hire from the same global talent pool.
Secondly, you're not accounting for communication ease and timezone differences. A huge amount of the world is cut out if you optimize for remote candidates within +-1-3 TZs.
Finally, the assertion that the average valley firm doesn't need a $200k+ in 2020 USD terms engineer is also not necessarily true. Demand for senior talent is significantly higher than for mid range and junior talent, because mid-large size firms want it (to bolster their senior roster and make it easier for teams to subdivide), and small firms absolutely need it (they simply do not have a senior engineer and need at least one).