If you read the quote in context he's not only talking about financial sustainability, he's talking about the social sustainability of how Uber uses a "gig econ

If you read the quote in context he's not only talking about financial sustainability, he's talking about the social sustainability of how Uber uses a "gig economy" model to avoid providing people who are de facto working for it with benefits, a point the Uber CEO acknowledges in the article with his proposal for gig economy companies to establish funds to provide those benefits.