For $6b, I would probably have erred on the side of caution and sold. However, maybe that's why I'm not founding multi billion dollar companies. A willingness to take on significant risk is a reasonably big part of being a founder. I respect his / their willingness to have faith in their company and business model, however flawed it may have been.
For a fairly popular example of some other founders who didn't accurately value their company :
For a fairly popular example of some other founders who didn't accurately value their company :
http://techcrunch.com/2010/09/29/google-excite/