A big part of his job is being a stock promoter. He is raising funds at a 500 billion dollar valuation, so it clearly works. Promise the moon (or Dyson sphere) is a proven way to generate hype and stay in the limelight.
It's not that crazy from a futurism perspective. If we become an interplanetary civilization, we will probably have things like this run our compute. I just wouldn't spend money investing in that future today...