That came from the layering armor part of the equipment chapter. I wish I could give you the quote I am thinking of but I do all my posting from work. Like I said though, King Conan doesn't seem to have his natural DR cut in half so I am guessing that you don't cut it.
This whole discussion came up because one of my players thinks that barbarian DR is somehow different and cannot be reduced by AP. Personally I think he is being mislead by what is essentially flavor text in the barbarian DR description, but I was wondering if other people have come to the same conclusions.