the heron spear isn't dangerous to mages, Mekeritrig used it on the glamour around the ark, fully depleted the weapon and then discarded it when it didn't succeed.
of course it the consult recovered it, rearmed it and put chorae in it after Shae broke the glamour, then that would explain things.
Also, isn't it just terribly interesting that the Nonmen warred for millenia inside the Ark and Ses and NC snuck into the Ark, searched it, and found the HS in a matter of days. That seems unlikely given the presumably incredible dimensions of the ship.