You're right, you're back is not supposed to be completely straight. The proper wording is that your lower back would be straighter if your hips are rolled farther forward (and doing what Danno instructs).

Compare the photos that Danno posted to the photos I did. You don't see any difference in the amount of curvature in the lower back? Maybe I'm missing something then. It appears to to me that a lot of riders plant their butts like they're on a barstool and bending their lower back and hunching over to make the most of being layed out and producing power - even when they stand to sprint.