The title makes it sound like they talk about how they did it so efficiently.

But all the info we get about that is

"The last step was to combine the four binary models into one multiclass model, as explained in the previous section, and use it to classify 1M new documents automatically. To do this, we simply went on the UI and uploaded a new list of documents."

Great intro to NLP article, but very light on the actual implementation details and dataset.