He can't. That was the whole point of the statement.
If 400 years ago, you told somebody that people would routinely cross the Atlantic by flying in metal birds larger than their ships, you'd have had a tough time keeping out of the nuthouse.
Heck, in my own lifetime, if you had gone up to someone in 1960, held a Pentium 4 chip in front of him, and told him how many transistors were in it, he'd bring out the coat with long arms, too.
It's entirely possible that remarkable new ways to do space flight may show up. If I knew what they were, I'd be a lot richer than I am.