Okay it’s time for a little history lesson.
Guns have actually been around for nearly 700 years. Less portable firearms like cannons and shit have been around even longer. It’s just that a lot of people tend to see them as more ‘modern’ weapons, thus leading to the Tiffany problem.
While samurai and their swords date all the way back to the Heian era, they were most prominent during the Sengoku and Edo periods, and most modern depictions of samurai in movies are from that time period.
While there were other early Japanese swords, the katana was invented around 1400, right around the beginning of the era of warring states. They stopped being used around the mid 1800s when being a samurai became illegal.
Therefore, even if this did happen, which I highly doubt, nerd boy here is dead wrong. If this were a western sword maybe it would be the child of a couple millennia. But a katana? That blade evolved separately from western swords. It’s made differently and if he really knew how to fight with the thing he’d know it’s used differently too. And the gun both predates the katana and outlasts it in terms of actual widespread usage.
This has been a history lesson.
Oh and p.s. you wouldn’t have a katana for judo practice, judo is a style of unarmed combat.