I think that although SAD and being a recluse are linked, they aren't the same thing. It's interesting that the reported amount of people with SAD in the US is so high compared to Japan [0] so I also did a little searching in Japanese. It seems that because of the availability and access to psychological treatment in Japan[1], the reported number of cases of SAD is possibly 5 times lower than the number given (0.7%)

[0] WHO World Mental Health Survey Consortium: 2004

[1]http://pax.moo.jp/sadjournal/2014/01/24/日本人は社会不安障害にならない？/