It's an interesting set of assumptions they made to arrive at that number, but piling up assumptions upon assumptions leads to a huge margin of error.... especially when we really dont know what conditions are necessary......earth is a sample size of one, so making assumptions based on just us is bound to have a HUGE margin of error.
But by far the most interesting aspect of the whole thing (for me) was that even if there are as many as three dozen of these civilizations, the average distance between them of 17,000 light years is far too much for us to know of their existence. So we should ask a different kind of question instead......."What is the necessary average distance between home planets of sentient civilizations for us to know of the existence of at least one other?" And I dont believe the SETI method will work, because there were assumptions made there also....not to mention the timeframe for detecting that kind of signal may be too small (in our case when we went digital our signal leak lessened significantly.)
About your very interesting speculation on the chances of evolution resulting in intelligence....isn't it true that dinosaur brains were getting larger and larger as the Cretaceous went on and they were approaching the intelligence of upper echelon mammals (as long as acquiring other traits of theirs, like being warm blooded, giving birth to live young and taking care of those young, as well as hunting in packs, like modern wolves do?) So perhaps evolving intelligence is more likely than we think- particularly when there are other highly intelligent animals, like elephants, dolphins, some parrots, chimps, bonobos, dogs, cats, pigs, etc. All of these have at least the intelligence of a 5 yr old.
I would make a contrary suggestion, that given billions of years for evolution to occur, that once multicellular life forms, it will inevitably lead to "intelligence"- and that there were other intelligent animals that came before humans and there are other intelligent animals that live with us today. This of course assumes that nothing occurs in the mean time to make the planet uninhabitable.
Their lower limit of 4 seems to be more along the lines of what I'm thinking....... because so-called "intelligent" species once they reach our level of development, probably have a higher propensity to destroy themselves. I strongly believe we are headed down that same dark road (but there will most probably be other highly intelligent species that come after us, unless in the process of destroying ourselves we also render the planet uninhabitable, which is a definite possibility.) I would also add one more requirement in addition to the ones you listed....the need for a proportionately large moon......its tidal influences were necessary to bring marine life onto land. The sun alone would not be enough. In addition our large moon shielded us from a ton of impactors that could have rendered our planet uninhabitable.