The anomaly itself does not seem to be disputed. It's true that when looking for arbitrary clusters and unusual patterns you can pretty much always find something -- though it's more of a Texas Sharpshooter fallacy [1] than the kind of multiple comparisons you see in e.g. medical research -- but the paper in question [2] mentions a whole bunch of alternative theories for why the particular clustering of a bunch of rocks in the Kuiper belt is what it is, and it doesn't seem very likely to me that none of those researchers ever thought "well, gee, maybe this is all just a fluke." In fact the competing hypotheses would be my main source of skepticism – maybe it's a planet, maybe it's an entirely different phenomenon.

[1] https://en.wikipedia.org/wiki/Texas_sharpshooter_fallacy

[2] http://iopscience.iop.org/article/10.3847/0004-6256/151/2/22...