The article clearly delineates why instead of one organ, the brain is now considered as two separate organs. The forebrain and hind brain don't have the same progenitor cells. If they don't originate from the same cells, how can they be considered one organ? They are just right next to each other, like the gallbladder and the liver.

Read the article. The hindbrain follows a parallel developmental path to the fore and mid brain, and does not branch off of the same starting progenitor stem cell. So the brain as "one organ" is composed of two separate tranches of stem cell, which is important for these researchers because developing hindbrain neurons and cells has been basically impossible apparently as they've been trying to produce them using what they assumed would be the correct progenitor cells that the fore and mid brain both start from.