To be honest...I'm more inclined to go with this story was FAKED intentionally to get readership/viewing the news agency up AND/OR for the reporter to make some quick cash. I'm also wondering how exactly this would be cost effective if you need to use chemicals to dissolve the bun. I'd bet that the chemicals themselves would probably cost more than making the bun itself. This leads to the other point, cardboard is quite thick. I'm pretty sure you can tell whether there's something wrong with the bun in this case.
As for China's food...though I'm Chinese, it leaves me wondering what exactly is going into our food, but that can be said to all other places too not just China.