“If you told me the Earth would only last a hundred years (i.e. won't last longer than that) .... It's a moot point since the Earth won't only last a hundred year (i.e. it will last longer).” At least that's what I got on the first reading.
I think I could kind-of make sense “it would increase the immediate priority of CFAR and decrease that of SIAI” under either hypothesis about what he means, though one interpretation would need to be more strained than the other.
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.
Here's the new thread for posting quotes, with the usual rules: