I've never seen a problem when they have been given cardboard. If I had seen a problem, I would tell everyone over and over again until everyone would be scared of using anything with cardboard!!
I have seen chins eat a little or it appeared as such. But, it doesn't taste good and they don't seem to keep on eating it. They turn the cardboard into little chips and are all over the place the next morning. So, I know that if they are eating it, they aren't eating much. Its texture is different from the carefresh or tissue paper or anything fluffy...so it has to be chewed up like the hay to be swallowed.
With that being said... If you see that the chin is eating the cardboard and keeps on eating it and isn't shredding it and making a mess, I would recommend not giving it anymore. You don't want any chin to eat too much at all, of course!