Musk says a lot of dumb things. I'm about as far as a Musk fan as you can be. But acting like rockets are going to be powered by clean energy in the foreseeable future is just wishful thinking.
it was already practical? The 20 years was just for the economics to work it out.
If you want to make that comparison watch how long commercial clean energy air travel will take. Then we can make prediction about rockets, which, mind you, escape freaking Earth's gravity
So were practical electric cars 20 years ago.