I never saw that date. Sorry about that. I saw the comment in a thread that also had recent comments and didn’t see it. But it makes the point stronger - if he knew about ion engines in 2015, he surely knows about them now.
I posted that to refute the notion that Musk is just a stupid rich guy who got lucky and actually doesn’t know much about his own technologies and just takes credit for what his engineers do. Specifically, that he’d never heard of ion engines when he said you can’t make a rocket powered only from electricity.
A better explanation was that a ‘rocket’ is generally considered to be something that launches from something else, while ion engines are used on deep space probes and satellites because they have very low impulse and need to run for a long time to generate velocity.
In fact, Musk was correct because a ‘rocket’ is defined as a cylindrical object that uses the exhaust of burning fuel to move. So now, you can’t power a rocket with electricity.