They didn't delete a part of container, they deleted a part of the docker file. Actually removing data from already built container image, as title suggests, would be much more impressive.
The final result is a 78% smaller docker image, not container. But the way they achieve it, is by running a container of the image, running the functional test suite, then remove everything that was unused when the test suite ran, creating a new image from the results of the removal.
I don't think the title is purposefully misleading, it's just incorrect by mistake (confusing image/container).
TBH, I thought that they would delete a part of a running container, something to do with de-allocating memory pages in runtime and all that low-level C magic, but now I see that it was just a really weird way to read this on my part.