His follow-up tweet:
This verifies what I said - that he was specifically talking about electric propulsion in terms of lifting off from Earth.
I have been following Musk for years, and listened to just about everything he’s said about rocketry. He knows what he’s talking about. No, he’s not a rocket engineer and I’m pretty sure he doesn’t do workaday math and detailed design. But he knows enough, and in enough detail, to oversee teams of engineers and guide the product strategy of a highly technical company, and to make decisions on technical issues his engineers present to him. He has made bets that other rocket engineers said were foolish, and he was correct. People who have worked with him almost universally say he’s incredibly intelligent and does his research.
If you think he is so ignorant that he doesn’t understand the basics of rocketry or what ion propulsion is and does, then you don’t know much about the guy or his knowledge level.