The bolt turned corporeal as soon as it reached the mirror image. It had to become corporeal to affect its target. While the anchoring line was the target, attacking it directly wouldn't accomplish anything. The line was essentially endless since it stretched into reality itself. In order to break the line itself, one had to break reality itself.
The goal of the attack was to detach the anchoring line from the mirror image. In order to accomplish that, the attack had to destroy the outermost layer of the mirror image. If the outermost layer were destroyed and replaced by a foreign force, the anchoring line would lose all possible points on which it could connect to the mirror image, detaching it.
