It's only partially true. While many could walk across the street and work for another Fortune 1000 company tomorrow or a well-funded startup in particular areas.

But, I think the point he was trying to make is that because of these mega-companies coalescing their people in certain cities, the economics of that takes a toll on everyone but the top executives who are already getting paid big cheddar + their enticements for being at the company: their stock awards which if there for even 5-10 years can amount to a huge amount. Enough for said ~managers~ [executives] to do the same sorts of hops globally as ~managers~ [executives] are interchangeable regardless of the product they produce.