Lerning by doing is the only way how it sticks 
If you want to broaden your docker knowledge, this self paced training is highly recommended.
A container is always created from a specific version of an image (even if a mutable tag like “latest” is used, it still points to the specific version of the image and not to the latest image the tag is pointing to) . In order to create a container from an updated image, you need to remove the old container, pull the new image (if the tag exists in the local cache, the updated image doesn’t get pulled) and create a new container based on the new image.
As far as I understand, the section of the Dockerhub description specifically covers major version upgrades, as it instructs to perform all minor version updates before the major update is performed. It smells like the image maintainer prepares us for breaking changes that justify this cumbersome migration method.
I share your opinion regarding the mixup of import and export. It is a good idea to raise an issue on Github for the doku fix.
What I realy like about taring the folders is, that the archive will retain ownership, permissions and a well known state of your data. You can recreate the starting point of your migration as often as necessary.