Jonouchi has just beaten Seto Kaiba in a duel during a tournament.
Keep in mind that Seto Kaiba doesn’t handle loss very well.
There would be a lot of in-character deficiencies with a fic like this. To put it bluntly, Jonouch is not on Kaiba’s level. Kaiba is a far superior duelist and Jonouchi is just not capable of beating him.
The line Atem uses for that last bit is “Ano Kaiba o taoshite mo okashiku nai hodo ni”, or more literally “[His abilities have grown] to the point where it wouldn’t be strange for him to beat even Kaiba!”
This near the end of Battle City. Let’s keep in mind that according to anime canon, Jounouchi has been playing this game for maybe half a year at most. Let’s also keep in mind that according to anime canon, Kaiba has been playing this game since he was adopted by Gozaburo, aka a number of years.
Even if you want to take this scene as “Jounouchi isn’t at Kaiba’s level YET” (which is pretty absurd if you go back and look at the four way duel where Jounouchi alternately manipulated or just plain outplayed everyone in the room to get the outcome he wanted) Jounouchi’s sheer rate of growth in the game means that it’s not a matter of “IF” he can beat Kaiba, it’s “WHEN”.
As far as Kaiba’s stocks suffering from a defeat at Jounouchi’s hands, there’s absolutely no evidence to suggest that losing a duel has ever had an impact in Kaiba Corp’s stock. The reason why Kaiba Corp was in peril (again, according only to anime canon) is because Kaiba outright abandoned it for at least a month after his first defeat at Atem’s hands—and this is in a private duel, mind you.
Kaiba loses to Atem on national television during Battle City with seemingly no ill effects on his company’s stock. The problems with Kaiba Corp during the Doma arc stemmed from a mixture of actual corporate warfare on Dartz’s end, and people thinking that somehow Duel Disks were capable of making holograms actually come to life.
Really, it’s more likely to assume the sole victim of a defeat at Jounouchi’s hands would be Kaiba’s pride.
Under the bludgeonings of chance
My head is bloody, but unbowed.