Not exactly sure what they meant, but maybe just that it's often hard to install a single Python package if the machine can't reach PyPI. If you download a tarball or wheel of some Python package on a different computer and transfer it to the isolated computer, there's a pretty good chance the package is going to have at least one third-party sub-dependency, in which case trying to install it with pip or setup.py will cause it to try to install more packages from PyPI.

Docker's a decent way to address this, since you install the dependencies when building the image rather than when running the container, so you can build the image elsewhere or through CI and run the container on any server without having to allow access to package management repo servers.