Sorry, but this is really terrible logic. I have first-hand experience with watching VC's pour literally hundreds of millions of dollars into a company that I was certain was never going to make it (and which has since been sold off for pennies on the dollar). All along, people kept arguing that I must be missing something, because people wouldn't invest that kind of money unless success was assured.
I'm not saying that the "good reason" will result in a proven product. I'm just saying in their perspective they have good reason to spend that amount of money. And also it's Sequoia and they're not stupid.