OS container versus application container

Question

So, the other day I read about OS / System Container vs Application Container here.

It mentions that Docker is an application container and

Any container that runs the OS is a system container.

Now I'm confused, because even to run applications in Docker, you need to have a base image, which can be any distribution. So, shouldn't this make Docker also an OS Container technology? Can someone point out the differences between OS vs Application Container technology?