The problem with the original argument (if I read it correctly) is that Bostrom assumes that the "growth and expansion sensibility" is a near certainty and that lack of evidence of this type of civilization, and the fact that we exist (as compared to having previously been 'harvested'), means that life does not exist elsewhere.
However, what everyone else in this thread has been saying, is that his conclusion (that no life exists elsewhere) is wrong because there are number of other possible explanations as to why we don't see evidence of this civilization. A quick summary of some of those arguments:
1. The expansion that Bostrom assumes is inevitable is actually physically impossible at the scale he describes.
2. It is possible that we are actually a by product of that expansion (DNA being the 'self-replicating' robot).
3. The expansion is happening, but hasn't reached us yet.
4. No other civilization has reached such an advanced stage yet (it is entirely possible that our 4.5 billion years was an extremely and uniquely fast time for evolution to intelligent life)
5. Super-intelligent civilizations come to conclusions that expansion is not a productive or useful undertaking.
While it is possible that life doesn't exist elsewhere (or at least intelligent life), it is by no means the natural conclusion to Bostrom's hypothesis.