Well, "open source Heroku" implies that they're compatible with Heroku's API. That means application release management, runtime configuration (config:set), application collaboration via "sharing"... From what I see in https://github.com/flynn/flynn/tree/master/demo, half of the API isn't there yet. We'll see how it goes over the next few months.

It's not actually like that, it is it's own product, but this helps you understand what it kinda does.