It isn't brainwashing, it is just exposure. The author does not state that it was anything being taught, anything being forced on him. Rather, it was the exposure to individuals who were being hurt by the politics he favored. Previously, he had been in exactly the sort of echo chamber that you are describing, and afterward, he had conversations with people outside of that bubble.
It is the same way that many people will be strongly against gay rights, until one of their relatives comes out. Knowing somebody who is personally hurt by an ideology gives somebody the kick that they need to reconsider the ideology as a whole.
It is the same way that many people will be strongly against gay rights, until one of their relatives comes out. Knowing somebody who is personally hurt by an ideology gives somebody the kick that they need to reconsider the ideology as a whole.