Wait a minute, the article says there’s no star within 8 AU? That doesn’t necessarily mean it’s a rogue planet, it could have a host star at 9 AU, the distance of Saturn from the Sun. What an I missing?
I didn't read the paper, but if they can measure its orbital eccentricity, they can see if it's gravitationally bound to that star. Like they did with ʻOumuamua.