One critical factor that wasn't mentioned in the article is life form morphology and environment. There is substantial evidence that dolphins may have mental capabilities similar to humans, but they obviously lack functional abilities similar to humans. And it's functional abilities that allow for a species to develop the technologies required for interstellar communication and travel, what we would typically think of an ETI being capable of, provided it's at or a little above our technological level. My point is, the fact that all SETI efforts so far have been unsuccessful does not necessarily indicate that there is a rarity of intelligence elsewhere (of course there's also debate on just how extensive a search must be to find ETI, but that's another matter), because "intelligence" does not necessarily mean "enabling technology" insofar as having at least one successful SETI.
One other issue that's been commented on, I've come across discussions before regarding the possibility that there may have been intelligent species in Earth's distant past but they either went extinct or "left" so long ago there wouldn't be any traces remaining of their existence. That's certainly possible, but IMHO very unlikely, the main reason being that the fossil record since the Cambrian is fairly complete, although there are still discoveries being made. Any intelligent species would have to possess a brain (relative to body size) much larger than any found in the fossil record, with their genetic cousins similarly equipped (just as present day primates are similar to humans), and nothing even close has ever been retrieved. The paleontological record seems to be quite clear in this respect, with life form complexity roughly proportional to geologic time, the earliest being the simplest, and brains increase in size to accommodate the increasing complexity of the nervous system.