That does not mean that all these images are available for s390x yet, but the infrastructure is in place to several architectures into official images, i.e. all official images are manifest lists. An earlier post explained multi-arch images and how to use it.
At this time, quite some official images are not just multi-arch enabled, but also carry s390x binaries.
This is obviously a huge step forward for usability on s390x: you can take an image like e.g. postgres and just use it, regardless of whether you are on x86, s390x, or ppc64le. Sticking with that example, everything from the image's anchor page https://hub.docker.com/_/postgres/ simply works. Also, having Z as part of the official images does not require you to change Dockerfiles on Z.
The way these official images are built is via https://github.com/docker-library/official-images . So cloning this repo and grepping for s390x gives a first overview of which images are also provided for s390x. I expect this soon to be very visible on Docker Store -- they have got an architecture checkbox for IBM Z already.
At the time of writing, this is the list of official images that are enabled for s390x:
Kudos to Tianon Gravi and friends like yosifkit at Infosiftr for enabling the entire official image structure, the Docker team for the Hub/Store an general infrastructure work, and Phil Estes (IBM) and team/community working on multi-arch support.