I think that this is describing a withdraw from society that is triggered by some unpleasant event - rejection from a prestigious school, loss of job, etc.
A relative of mine lost her first child after a long, painful genetic illness similar to PKU. Afterwards, she divorced her husband, moved back in with her mother, and has rarely left the house over the past ten years. I would describe it as depression and social anxiety and other conditions that were triggered by the child's long, painful death. It is not AS.
Japan is a social pressure cooker in many ways, and they had a severe recession that lasted for over a decade. Many people experienced psychological trauma when the economy contracted, and I would put Hikikomori as a reactive behavioral syndrome. Maybe the people that do react that way are different somehow from the general public, but they seemed fairly NT prior to whatever crisis triggered their reclusiveness.