To be honest, I think the approach she took is a lot more reasonable. The whole point of building a product business is that your revenue scales differently from your expenses. Your example has an implicit assumption that Google is somehow artificially regulating demand so they can keep their profit margin at 20%. The profit margin on Gmail could be 5% or 99%. Who knows?
No argument here. The only reason I went down this road was because I figured any guesses I made about actual click rates of users would be mostly based on a sample of 1 and thus even more laughably off than guessing the minimum viable earnings...of course, this is why one should never be ashamed to say you don't know. It's one thing to speculate and exhibit your thinking process, but when you start getting confident about it it's all too easy to slip in bullshit artistry, and I'm sure they'd rather avoid that trait in hiring.
