To you first point, there's probably no example of a Buildpack because (unlike Dockerfile) most Buildpack users don't write their own. They are reuseable. I'm not sure why the author didn't include an example of running a buildpack against an app though.

To your second point, Buildpacks are written in code too. But unlike Dockerfile, they use real programming languages that you can write tests for.

For example: https://github.com/paketo-buildpacks