Content based on Interactive Book type: Why was upload only successful after installing the Interactive Book sample content?

I have an H5P content (not made by me) that I tried to upload to a website using the most recent Wordpress plug-in. Let's call this content foo.h5p. The upload didn't work, instead there was a message saying the Interactive Book content type was missing. This is the content type foo.h5p was built upon. When you unzip it, there's a folder named H5P.InteractiveBook-1.2 inside.

Then I uploaded the Interactive Book sample content from h5p.org. It's the content about berries. The upload worked and when I tried to upload foo.h5p one more time, I also succeded.

How is this possible? I thought H5P contents contained all libraries they need. What exactly made the second attempt work? Did the content need 'external' files, or does it rely on some database entry?

Thanks
Pida