Sort of true. In practice, it's usually a bit less that twice - pentodes are more efficient. You can get the same efficiency from a triode if you push enough grid current. In both cases, the source impedance is too high to be useful unless you use plenty of feedback. Vandeveen uses plate-to-grid feedback around the output stage, making it behave as a current-controlled voltage source - called a transresistance.
The driver acts as a voltage-controlled current source - called a transconductance. Vanderveen adds a local feedback to make that more linear. His driver cathode follower also is a feedback stage.
Lots of feedback means you need lots of open-loop gain, and therefor more tubes - the Vanderveen amp has four elements compared to two in most SETs. That eats up the cost savings from increased efficiency. No free lunch, no magic bullets.