This was a very cool read. Programmers were programmers even back in the day of Mark I.
It is cool to see that they dabbled in natural language processing back then. This is years before Eliza and they were working on generating English prose based on English grammar. Very impressive!
The music generation program they wrote is equally impressive. The recording that was playing shows that they were adept enough to time events in the computer so good that they could playback songs. This was back in the early 1950s.
Decker is a "multimedia sketchpad". It's a drawing program, a database, and a rapid prototyping environment. A "deck" can be exported as a self-contained, self-executing .html file (as seen above), and there's also a non-browser-based native application. It uses a functional scripting language named Lil which incorporates ideas from the APL family. You can learn more and play with some additional examples here:
Aside from the scripting language, a major innovation Decker has over HyperCard is the concept of "Contraptions": user-defined interactive widgets that can be reused by simply copying and pasting them between decks. If you can make a card, you can make a contraption, and then you can share it as a new building block:
(https://news.ycombinator.com/item?id=40948325)