
The word coward was used 5 times total in the novel, and it was Stratt accusing him of being a coward twice before he ever called himself one, denegrating himself AFTER recalling the memory of his selection for the mission.
Like I said, he did certainly have a cowardly act when faced with death. One act does not make a person a coward, and for the whole story up to AND after that point, he dives headlong into danger.
Maybe the literal word wasn't used - but I'm failing to think of a single other cowardly act from Grace in the whole novel. I'd be happy to reread any section that you think fits your narrative, but for now I really strongly disagree and had the opposite takeaway.