How is it misleading? The entire post reads like a PR piece for their company, but the title seems faithful.
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).
What tooling does one use to determine this? How can you see what files are being hit or not? Ptrace?
Edit; I RTFA and think that’s the product they sell. But how would one go about this without their product?