That's deep. Seriously. You should unpack that or give your take on it, because I'd be interested to hear your take.
Oh man. There's so many directions you could take it. I'd say it boils down to John 14:6, no man can come to the Father except through Jesus. Does it matter to the man who didn't come through Jesus that he lived a good and moral life? No. Because he still missed God.
When I first read it, it made me think of when people attempt something and fail, or they reach the end of a road they've chosen to take, and it does not have the result intended. The LAST thing that person wants to hear is, "Well, maybe if you hadn't done this...or maybe if you'd tried that instead." All that matters is they failed, and they have to start over again. Of course, since this was written by C.S. Lewis, your interpretation of the quote is probably spot-on.