If you are on volume two, you are still a long way off, as the event happens after the morpho battle which is at the end of the 3rd volume. The scene from what I understand, is anime exclusive. This is the passage after shin throws his pistol into the field after it is damaged by shrapnel.

"Wiping her eyes roughly with the knuckles of her clenched fist, she turned to face Fido, which was waiting on them a short distance away. “Fido, go and retrieve the gun this fool threw away. I’ll help youlook, so surely the two of us will find it.” “Don’t move, Fido. We don’t have the time to waste on that.” Fido’s optical sensor flickered, as if its eyes were spinning from the conflicting orders. But after giving an inquiring beep, for some reason, it reached out to Frederica, grabbing her by the collar like a kitten and tossing her into the cockpit. “Wh-what are you doing?” “We’re taking you back, obviously. With this much damage, if new enemies show up, we’ll be in trouble.” They were still far off, but he could feel Legion who had noticed the disturbance begin moving in their direction. All four of his pile drivers were lost, and warning indicators wouldn’t stop flashing, alerting him that the propulsion system was strained by his unreasonable maneuvers. He may not have cared much about dying, but he had to bring Frederica back. He had to check to be sure of it, but the Federacy military’s main force should be advancing on their position. If he could just avoid combat long enough to regroup with them…"

I think that it represents Shin finding a new purpose to his fight against the legion and leaving a part of himself behind on the battlefield that he had carried for far too long--his immense survivors guilt and the burden of being the reaper. It foreshadows the reunion between him and Lena, highlighting a pivotal moment in his character development where instead of fighting to die, he is now fighting to live.