So, in that example, anywhere outside of SF in the South Bay isn't an option. Beyond highly concentrated cities like Manhattan where there's a huge confluence of jobs in a particular industry, job mobility causes issues in any case (especially once the jobs of partners and kids' schools are involved) in any case. It's rare industries and location where you can generally change jobs without commute being a factor.
They are saying that many, in fact most, 'local' jobs exist with relatively little opportunity cost for change.
Contrast this with a hypothetical future of (E.G) New York, where in 30-50 years they've gone full caves of steel: banishing cars to the edge and having a multi-layer dense mesh of rapid electric motor powered transit and living which intermingles living, working, recreational and (deep underground) industrial productivity. There the opportunity costs might also be the same, but the cost (in time) of migrating might be less, and favor walking on moving belts for a combination of speed and exercise.