Kind of, but not really.

Auroras dont necessarily need a stars radiation. Any old radiation will do, so long as there are charged particles floating around. Jupiter, for example, has gigantic continuous aurora around the magnetic poles. If auroras only came from the sun, and the earth is much closer to the sun than Jupiter, wouldn't earth have a bigger aurora than Jupiter?

No, obviously. The size of the aurora depends on the size of the magnetic field interacting with charged particles and the number of those charged particles.

In the case of supermassive planets like Jupiter and this rogue planet, they produce way more of their own radiation than they recieve from the sun or space. This rogue "planet" in particular is so massive that it could actually fuse deuterium down in the core just with the pressures and temperatures of gravity crushing all that matter down. If you pumped enough hydrogen in there to quadruple the mass, it would probably ignite into a star quite comparable to our sun.

For that reason, it's better to think of this as more of a baby star that didn't quite eat enough wheaties than a planet in the traditional sense we think of here in our solar system.

With the crazy physics that come with suns and near dwarfs with similar mass, it's no surprise that it generates a titanic magnetic field, and as a bonus, it produces its own radiation. It creates all the necessary ingredients it needs to make it's own spectacular auroras with no actual outside interaction.

Tl;dr it makes it's own aurora