Reader AT actually met Tom Shanks, the chief programmer behind the ACS Atlases, and corrects my understanding of how the ACS team put it together:
Contrary to what you assume in your post, Tom Shanks did not hack his
atlas into an Apple II. ACS was rather professional in their IT. The
worldwide city database with longitude and latitude they had licensed
from on of the big atlas (map atlas) publishers, if I remember correctly
Rand McNally. The timezone history data they had collected from numerous
published sources..., and partly also from field research and grass root contributions
by their own astrology service clients.
The reader also gave me the story about an ongoing effort to extend the tzinfo database, and the provenance of Astrolabe's alleged copyrights. (Note that this is the precise, legal meaning of "alleged:" in a civil complaint, just as in a criminal complaint, the parties "allege" each fact in their filings.) If the reader gives me permission, I'll post some of this information.