The author's central thesis - that aliens who are advanced enough to send an invasion to earth but not advanced enough to get the resources they need by other means is highly implausible - doesn't rest on the throwaway points with which you're quibbling.
Throwaway points as to how likely it is for advanced aliens to even exist and whether they might be willing to trade with us? He's attempting to refute all possible motivations that aliens might have to communicate with us.
If you don't understand enough to know that the extremely small probabilities that have been argued for abiogenesis can completely overwhelm the large number of planets in the universe--so much that you dismiss it out of hand--this is a bad sign of the author's competence on this topic.
I don't think anyone is dismissing abiogenesis, just that the likelyhood of life evolving on a planet, then evolving to be intelligent, then developing technology way outside our knowledge of the world (like FTL), then deciding to go out and explore in our small part of the galaxy (and with 400 billion stars in the Milky Way and billions more galaxies, I do mean small as in a cluster a few hundred light years in radius to detect human radio signatures at the very least), and THEN, deciding to come down and meet us/trade with us/invade us/destroy us/take our resources (which are very human things to do. considering how different psychologies of cultures can be on this planet, I don't think these are the only options).
I don't think anyone disputes that any of those things on the list are "possible," just the odds stacking up in our favor.
