You raise a good point. Upon re-reading, portability to a different cloud is probably not the point. It's more likely about being on the cloud you're already one but having the choice to take only parts of the platform, instead of taking Amazon everything at every layer of the stack. So you're still on Amazon or whoever's cloud, but you may not use (for example) their database if you like some other database.

In other words, it's not about moving your entire stack off Amazon's cloud, it's about moving parts of your stack off Amazon's software even if all of it may still run inside Amazon's cloud.