It's very true in my experience, and we've looked pretty hard at all the options. Thanks for the offer of the slides, I have first hand experience with this at scale.
It certainly depends a lot on how sophisticated your autoscaling is and how closely you're able to follow demand to limit waste, how well you can manage per-host utilization, whether you are CPU or memory bound, and lots of other factors. But even at truly massive scale the cloud hosting option can be very competitive without nearly as much management overhead.