It seems obvious from the dialogue that it was understood that having the Lakota use its quantumn torpedoes would destroy the Defiant without question
The Defiant was hit by torpedoes on other occasions, and was not destroyed "without question". It's true that, given its size, it's not going to take as many high-powered weapon hits to destroy the Defiant as it would other, larger ships... but at the same time, the Defiant is built compact and heavily armored, without inherent hull weaknesses that you get from having windows all over it like a Galaxy does. It's a very tough ship for its size, not a flimsy wet tissue.
It's also a universally acknowledged fact in canon that the Defiant has the most firepower of any Federation ship at the time of its first deployment. Honestly, with the number of times something to that effect is said on screen (even by Gul Dukat at one point) it's amazing people still doubt it.