I think it's possible (although not a foregone conclusion) that in fact much of the filter is from what he terms 'discrete' events. That is, having the right sort of sun, the right-sized planet at the ideal distance from this sun, with a substantial moon to act as a gyroscope, active tectonics to recycle elements that would otherwise get locked up, etc.
I've not, for example, seen any conclusive estimates on how likely it is that an otherwise earth-like planet would get a moon to act as a stabilizer for axial tilt (which itself is probably important for complex life).
Also, his assumption that life could spread at nearly the speed of light may be overly optimistic. If, by contrast, one assumes that light can spread at only 1/10th the speed of light, and that the earliest star-faring life could have arisen was a billion years ago (due to as the time needed for a second-generation yellow star to brighten enough to keep an earth-like planet warm after its conversion to an oxygen/nitrogen atmosphere), then one only needs to explain why star-faring life has not yet arisen in relatively nearby galaxies, rather than why it has not yet arisen anywhere in the visible universe. This is a much less daunting hurdle.
I've not, for example, seen any conclusive estimates on how likely it is that an otherwise earth-like planet would get a moon to act as a stabilizer for axial tilt (which itself is probably important for complex life).