I think the underlying premise of the question -- e.g., "Why was this sample bias not thoroughly addressed before?" -- is somewhat incorrect. Previous papers, including papers by those making the "Planet 9" claim, have attempted to address sample biases; a secondary issue is that the new paper uses data unavailable to previous studies (and also ignores some of the previous data).
To begin with, the original paper by Batygin & Brown (2016) does include a brief discussion of the possible effects of selection biases (in Section 2). This was, admittedly, pretty cursory, but they have followed it up with more extensive analyses (Brown 2017; Brown & Batygin 2019). From the abstract of the latter paper: "To determine if observational bias can be the cause of these apparent clusterings, we develop a rigorous method of quantifying the observational biases in the observations of longitude of perihelion and orbital pole position. From this now more complete understanding of the biases, we calculate that the probability that these distant KBOs would be clustered as strongly as observed in both longitude of perihelion and in orbital pole position is only 0.2%."
So Napier et al. are not, as you suppose, the first to consider selection biases (as they indeed acknowledge). What, then, is the difference? Part of it is that Napier et al. argue for performing an analysis using detailed simulation taking full account of individual survey characteristics if they are known. In order to do this, they focus on objects found in three recent surveys. This means that they deliberately ignore the 6 objects used for the original claim of Batygin & Brown: "The six ETNOs considered in the Batygin & Brown (2016) (BB16) analysis were discovered in an assortment of surveys with unknown or unpublished selection functions, making it difficult to establish that the observed angular clustering was indeed of physical origin." Instead, they use 14 objects "detected by three independent surveys with characterized selection functions, all published since BB16."
(Note that the Batygin & Brown 2019 bias analysis also used 14 objects, but not the same 14 -- they included the original 6 objects that Napier et al. exclude, but did not have access to objects detected after their study, which Napier et al. do. We are in a situation where there are very few data points, and there is the potential for divergent results based on the small number statistics of different studies.)
Eric Jensen has already posted links to some Twitter discussions by Batygin on this topic; I can point you to a Twitter thread by the other original author (Mike Brown), as well as a recent blog post by him which tries to understand why they and Napier et al. get such different results.