It didn't have to be a mythical engine that ignores the most basic laws of physics. Just like with Rearden's steel, a more optimized version is often sufficient to change the world.

Both Elon and Bezos have significantly decreased cost to orbit and are very close to full reusability. This is alongside their other feats like a global logistic network that gets pretty much any product in the world to your doorstep in 1-2 days, self-driving cars, neural implants that enable mind-controlled computing. If that were in the novel, no one would believe it.