Is the existence of a planet so easily falsifiable? It hasn't been so long since the Planet Nine hypothesis started going around, and while we've observationall

Is the existence of a planet so easily falsifiable? It hasn't been so long since the Planet Nine hypothesis started going around, and while we've observationally ruled out a big chunk of the original parameter space, there's still lots of room for a big dark dwarf planet to be floating around out there. It doesn't seem so different from how we've gradually been ruling out the parameter space for dark-matter observations.