Goh did catch it. It was his Poké Ball, and the capture was registered on his Pokédex. Ash just helped to throw the Ball because it was so heavy. Even if he's never going to use it, he still technically owns it.
Key word technically. There's no way Go would use it even if he had the chance (and not just because he rarely uses the pokemon he does catch), and a Snorunt's chance in aech eee double hockey sticks Eterantus would actually listen to him. It's been described as evil and just being out in the world unrestrained is a crisis level threat that took two legendary pokemon to weaken. Not even Unova Ash or Team Rocket would be stupid enough to mess with that after witnessing the Darkest Day.