Almost immediately his claim was met with skepticism, but Nakano seemed to have an answer for everything! For example, people disputed that Magikarp couldn't injure ghost Pokemon using Flail, to which Nakano replied he exhausted Flail to use Struggle to hurt them. It's also been pointed out that Magikarp is capable of learning Z-Moves which boosts his effectiveness from useless to "meh".
I'd love to see a stream of him doing this, but I'll take him at his word with the photos. I don't see anyone ever trying this because it would take so incredibly long and be so frustrating! Congrats Nakano!