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
The current deal that I've read for Uber's Dallas expansion is that they currently are leasing about 1/3rd of a brand new skyscraper that was just built, have plans to lease 90% of a brand new skyscraper that is currently being built, and also outright purchased several plots of land where they are planning on building office building that they will own.
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.
