I haven't seen the original research myself to judge, but it sounds like they're being careful about these effects. Looking at their discussion on the main page

I haven't seen the original research myself to judge, but it sounds like they're being careful about these effects. Looking at their discussion on the main page of http://www.findplanetnine.com/ http://www.findplanetnine.com/, the claim seems to be that this isn't "we spotted six out of the solar system's zillions of objects that formed a cluster" but rather "the unique six known objects within a certain range of orbital parameters all form a cluster in a different parameter". More specifically, quoting from that page, "all Kuiper belt objects with orbits that do not veer into inter-planetary space and spend longer than approximately 2000 years to complete a single revolution around the Sun... trace out elliptical paths that point into approximately the same direction in physical space, and lie in approximately the same plane." That could be just an observational coincidence, and that's where they claim the statistical figure that you've cited. I have no idea to what extent those figures account for the multiple comparisons issue, but the range of orbital parameters that I've quoted here doesn't look very fine-tuned to me. (The upper bound certainly isn't, anyway!) So it's just a question of how the lower bound was chosen. And on some level, I'm not even sure that that's an issue: an observation that's based on "the longest N bound orbits all show this pattern" feels meaningful to me even if I'm not sure in advance how big N is going to be.