The code is being open-sourced as MIT-licensed libraries. The libraries mentioned in the article are Brigadier [0], a command parser and dispatcher, and Data Fixer Upper [1], which is used to migrate older chunk data to newer versions of Minecraft.
Apparently more libraries are in the pipeline to be open-sourced alone these two. One possibility is Blaze3D, which is a rewrite of the rendering engine that is planned on being included in Minecraft 1.14.
[0]: https://github.com/Mojang/brigadier
[1]: https://github.com/Mojang/DataFixerUpper