Uh read the quote
The relevent text is right there in his post. Assuming X to infinity = X is a pretty big blunder. One of the main reason for expanding into space is because it keeps us from being limited to Earth's resources.
Others include:
1) The evolutionary based maxim diversify or go extinct ie don't keep all your eggs in one basket
2) Earth's habitibility is limited by solar evolution.
3) Earths habitibility could be limted on much shorter scales for many reasons.
4) Expansion into space would give us more potenial to protect Earth (related to above).
5) It sounds like fun.
6) The thrill of scientific and general discovery.
I think the odds are very good that the lifetime of technological civilizations that don't expand into space is short.
Regards,
Mark