If FTL is not possible, this makes for a very good reason to conquer the nearest habitable planets. If they are overcrowded and able to conquer Earth in reasonable (for them) time-frame, I don't see any reason why they wouldn't want to. That of course is only if "habitable" means the same thing to them as to us (liquid water, oxygen etc.).
You can come up with scenarios to justify anything, of course. The blog post seems to be discussing aliens as depicted in science fiction which almost always need some kind of handwaving ftl drive for narrative reasons. I was just suggesting the technological infeasibility of actual interstellar travel to be a more likely reason that we haven't or wouldn't encounter aliens, than their considering us beneath their contempt.
