That's a nonsequitur. The California coast is pretty dense, the rest of the state has large amounts of empty space and national parks. Nobody would build a high-speed rail through Yosemite or Death Valley.
https://en.wikipedia.org/wiki/Demographics_of_Taiwan#/media/...
vs.
https://commons.wikimedia.org/wiki/File:California_populatio...
The reddest areas on the California map would be orange on the Taiwan map.
A San Jose to Bakersfield route (comparable in length to the Taiwan route) has a huge low-population density area around the middle, so basically there's no prospect of using the rail for shorter trips. And the number of daily trips between San Jose and Bakersfield is likely to be limited.
The post author compared San Jose to Bakersfield, as it's the same length as the Taiwan connection.
Yet it detoured to stop in farmland.