Oh I don't know...

even using their numbers:
biologically active rocks per solar system = 1.5* 10^7 over 10GYr
* p.capture=10^-6
* p.impact=10^-4
* p.seed = (outside of scope of paper)
if the probability of a single rock successfully seeding a world after impacting is 100% (!) and the cluster has 1000 stars then there might be one successful seeding event in a cluster, over the 10Gyr period.

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plenty of woo, at the hotel hoagaland...