IMO there's no difference between a trained domestic engineer and a trained foreign engineer. Foreign vs domestic isn't a meaningful difference.
However. Existing tech hubs got their cachet for having the best and the brightest, which in turn draws in the best and the brightest. Companies have to pay more for talent on average in hubs because the talent has self-selected for a high concentration of great folks.
That doesn't mean there isn't an assortment of great folks elsewhere who just didn't feel like moving, but IMO, more than likely, the majority of the great folks are already in tech hubs. I think the risk is overstated.