Ah. Hmmm. Neither did I, actually. In RL cultures where duels were regimented to some extent (England and France from the Middle Ages through Victoria's reign), there were two "versions" - the duel with swords, and the duel with pistols. In the sword duels, disarm was accepted and usually ended the duel (unless one of the duelists simply wished to kill the opposite party.... a whole OTHER thing); in pistol duels, "disarming" generally took the form of the better shot literally disarming the other participant, by choosing to wound the other's pistol arm rather than go for the kill. This of course put the loser in a bad light, and for quite a long while, but kept them both from some certain repercussions. There was a whole "culture" of the duel....
But you would still be disarming the opponent via skill with an equal weapon, not shouting it out of his hand with magic.
Eh, I'm of the opinion that the use of the Thu'um in the duel was strictly symbolic anyway, since the general consensus seems to be that Ulfric would've beaten Torygg without it anyway. I'm pretty sure that Ulfric thinks that, lol, and IIRC doesn't he even say something along the lines of sending a message by using the Thu'um, or was that with regard to the challenge itself? I can't remember.
At any rate as a symbolic thing, Disarm might've been even more appropriate, but IMO it would've looked far more dishonorable and might even have lost him some support and street cred ("did you hear? Ulfric had to shout the sword out of Torygg's hand in order to beat him" - not good, lol).