The compiler is free to use for GPLv2 projects. If you aren’t open yourself, it looks like you have to cut a deal to use Cheerp with its maker, Learning Technologies.
The Hello World example is, understandably, a little underwhelming, but does show some special features that allow direct access to the browser. They do point out that you can ignore that and use things like
cout if you aren’t worried about performance. A better place to start if you want to do anything serious is the pong game example.