Heroku apps run in a hyper scale cloud, sure, but they abstract away all of that. When you "graduate" from Heroku you have to move your entire app and re-architect the entire thing.
The idea behind this effort is to put you in a cloud native hyper cloud environment of your choice to start with and make that just as easy as Heroku. The big difference will be when you "graduate" - you'll already be in AWS or what have you, and under the covers isn't a black box but is a well formed set of GitLab primitives that you can now continue to use and expand upon.