Yeah - but they don't have to be much smaller.

Let's use the simple example you reference from the paper. If we just use the mean as point estimates, then indeed we obtain an extremely low probability that there's no other intelligences:

  def prob_of_no_intelligence_in_galaxy(p):
      prob_of_intelligence = pow(p, 9)
      planets = 100e9
      return pow(1-prob_of_intelligence, planets)

  prob_of_no_intelligence_in_galaxy(0.1) -> 3.720086311124783e-44

However, there is already a 82% chance of an empty galaxy if the parameter p is halved:

  prob_of_no_intelligence_in_galaxy(0.05) -> 0.8225792614407508

Now of course halving every probability is a lot, however now there's no single or few events that have a very low probability of happening. The Great Filter disappears.

See also my other comment here: https://news.ycombinator.com/item?id=25811359