If he was serious about mass transit there are existing, proven technologies he could get behind with his billions, rather than hyperloop. Instead he proposes a technology that is entrancing to people who don’t know enough about engineering and physics, and that is exciting. Unfortunately the tech isn’t workable, it’s just exciting, but it could take a decade or more for that to become apparent to most people.
Wikipedia has an excellent article which lays out the history of the concept going back to the 18th century. https://en.m.wikipedia.org/wiki/Vactrain
I love the understatement of “prohibitively expensive” too, very droll. Any experimentalist who has worked with strong vacuums knows how absurd the idea of a hyperloop is at this point. The expense, the maintence costs, the airlocks, the pumps, all would be an incredible expense that could never be recouped in the builder’s lifetime. The actual tunneling would have to be totally revolutionary too, with incredibly long, nearly straight sections. The turning radius would have to be on the order of of a commercial airliner’s turning radius, which is absurd.
Credit to the man though, it hits that sweet spot requiring specialist knowledge to see through the hype, much like promises of nuclear fusion as a major power source within a couple of decades. The big idea is so attractive, and the problems all involve technical details. It’s not quite a con, but it’s close.