Stomach is not where the thickest part of armor should be and kitchen knife is better against armored opponents than a sword is. In real life there is no such thing as damage numbers so a kitchen knife is as much a one-hit-kill of a weapon as a sword, gun or cannon. If it hits the vitals, that is.
And stomach not only is not one of the critical vitals (you can die rather easily from being pierced in certain depth and parts of it, but not all), but it also requires more mobility than the rest of the body. A thick armor on the stomach means you cannot move. Like, literally. Heavy jousting armors were stiff on the stomach, and the knights couldn't even get up on their own once they fell off the horse (onto which they couldn't get without help either).
If you'll look at
this armor, you'll notice that around the area that Aymeric was pierced the armor is actually divided into the top and bottom, with the bottom being a sort of plate skirt. If you have the opportunity to stick something between the bottom and top, you can very much hurt the person.
Of course...if your weapon can pierce the chain mail that is traditionally worn under plate armor, but we don't know whether Aymeric wears it too.
Ultimately a kitchen knife is better to deal with a heavily armored enemy than a sword...but in reality...you're not likely to kill anyone with either. Attacking around the neck where it's easiest to slide past the multiple layers of armor is your best bet...or just use blunt weapon. So if we accept suspension of belief as for why arrows can kill heavily armored enemies...a dagger is the least problem here.