They use aws as infrastructure but aren't inseparably welded to it. They could presumably switch to hosting elsewhere without changing their product itself.
You can see them as abstractions of AWS in some ways. An average consumer wouldn't go through the trouble of dealing with AWS to backup their files and even for developers heroku provides a nice level of abstraction
Agreed, that's the way I've seen these companies. AWS has truly become a layer that powers a huge part of the applications on the web.
And also many companies are built on top of Heroku...