I don't think either of you are getting it right. I'm not familiar with the context of this particular quote, but knowing it's from Mencken, he's clearly referring to various idealistic busybodies and their grand (and typically disastrously unsound) plans to solve the world's problems. The quote is directed against idealists who assume moral high ground and scoff at those who question their designs.
Ah, so it's about whether a plan meets some absolute standard, rather than which plan is best, and the moral is that just because I don't know of a plan that meets standard X is no reason to think your plan will - in fact the reverse.
A monthly thread for posting rationality-related quotes you've seen recently (or had stored in your quotesfile for ages).