Maybe being a high school teacher is simply more pleasant than working in tech to some people? I don't think your example shows there is an untapped talent pool.

Of course, in general, you can make a job more attractive (raise salaries, roll out red carpets, install slides...), and you will attract more people. That doesn't prove those people were an untapped talent pool.

Presumably there is a price that would make a high school teacher consider working in tech again. That doesn't imply companies should be willing to pay that price.