The idea is that if Earth lasts at least a hundred years, (if that's a given), then the possibility of a uFAI in that timespan severely decreases -- so SIAI (which seeks to prevent a uFAI by building a FAI) is less of an immediate priority and it becomes a higher priority to develop CFAR that will increase the public's rationality for the future generations, so that the future generations don't launch a uFAI.
(The other interpretation would be “If the Earth is going to only last a hundred years, then there's not much point in trying to make a FAI since in the long-term we're screwed anyway, and raising the sanity waterline will make us enjoy more what time there is left.)
EDIT: Also, if your interpretation is correct, by saying that the Earth won't last 100 years he's either admitting defeat (i.e. saying that an uFAI will be built) or saying that even a FAI would destroy the Earth within 100 years (which sounds unlikely to me -- even if the CEV of humanity would eventually want to do that, I guess it would take more than 100 years to terraform another place for us to live and for us all to move there).
Here's the new thread for posting quotes, with the usual rules: