as an American who worked in Silicon Valley for years, but is not from there, i'd say it's also "captive real estate customers" created by H1-B people.
He said what you surmised: while there are plenty of qualified American engineers in the United States, citing Motorola headcount reductions as an example, it was a challenge to convince them to move to California's cost of living.
To recap: 1. Move to the Bay Area circa 1997 2. Buy houses 3. Profit enormously in the 2020s
Mind you, I had no idea...