From there he taught them harmonizing the chorales. But not supplying the three lower parts, as usual. Instead he wrote the bass for them and they only had to supply the alto and the tenor.
It makes sense. When the bass is given, the process is already half finished. All the students had to do was to see what kind of harmony did the bass notes imply and from there to add the two missing parts.
Finally, he asked them to create the bass themselves. This means they were ready to supply the choral tune with the three lower parts.
Apparently Bach even often composed in this order. In his autograph of the Orgelbuchlein chorales preludes, often the bass notes are more prominent. The alto and the tenor appear to be squeezed in later on.
Do you see what it means? It means that Bach already had a piece finished in his mind when he created the bass line.
Try supplying the alto and the tenor parts for this chorale and you will find out how much easier it is to work in this order as opposed to creating all three lower parts right from the beginning.