I wonder if the influence of the rest of the solar system wouldn't impact it. Consider: the reason we think there is anything out there is because we see a lot

I wonder if the influence of the rest of the solar system wouldn't impact it. Consider: the reason we think there is anything out there is because we see a lot of bodies that aren't following the expected pattern. For every action, there is an equal and opposite reaction, so perhaps those out-of-order bodies also pulled Planet 9 closer to the standard plane. Repeat a few billion times over a few billion years, each little tug moving it more in-line (and the rest of the solar system out-of-line). This still depends on it starting relatively close to the same plane, but there may be selection bias involved. Those bodies that met with the solar system and weren't aligned well with the plane may have been more likely to escape again. The one (or ones) remaining are the ones that were well-aligned.