A 51% attack would be disastrous even if they only double spent one satoshi. It's the loss of confidence in the technology that's the harm. Bitcoin relies on people believing in it to be a secure store of value.
Yeah, spending a single satoshi would be as easy as double-spending 10000 BTC. I don't know what background the author of the post has, but when high-profile people behind Bitcoin's development claim that a 51% attack is a potential danger, I do believe them. From what I understand, it really poses a certain threat to Bitcoin!