Earlier quoted context omitted.
The difference is that this isn't some company with an existing footprint in a city saying "hey we're gonna hire some more people for our office here". In both the case of Chicago and especially Dallas, Uber is making significant real estate commitments to go along with these hiring plans. You don't buy up an entire skyscraper office building (and several plots of land around it) and open an entirely new office unles…
I don't believe Uber is buying a building, rather leasing out space, which can always be re-negotiated. There have many examples of companies promising a certain number of jobs and never fulfilling them, because who is looking. For example, Foxcon. Also, Uber once committed to build a HQ in Oakland and signed a huge lease. Soon after, they abandoned that plan and started building a brand new HQ in SF
edit: After reading a few different articles it seems like every article says a different story about how much space Uber is committing to, so maybe you're right, it does sound a little wishy-washy.