I think there is a bit of a flaw in the way this indeed survey and others identify national tech hubs.
According to this survey, the top tech hubs have the highest percentage of their job listings advertising for tech jobs. This disregards the actual total number of tech jobs.
For example, Austin is considered a tech hub in this last while Dallas is not, but Dallas has way more tech jobs than Austin, which is a smaller city. The difference is that Dallas has a very diversified economy, so the percentage of job listings for tech is lower.
Personally, I'm more interested in the tech hubs that are part of a larger diversified market. Just like a diversified portfolio protects you from losing money from fluctuations in the market, a diverse economy protects your ability to find a local job when your industry takes a downturn.