From the website: "The Great Silence implies that one or more of these steps are very improbable; there is a "Great Filter""

The paper argues against this in that you don't need a few very improbable. In fact, there are many parameter choices where none of the probabilities in the Drake equation are particularly low, yet the resulting number of intelligences is still staggeringly small.

tl;dr: All parameters can be a bit lower. Then you don't need any events that are extremely improbable.