It was both. And it's a fantasy series, you shouldn't be too concentrated on realism, especially about Sam not being skilled enough. It's, again, a symbolism, that you claim we don't see - that a chubby lubber without any real skills but with a brave heart can be the decisive man in the battle for the realm's fate. In Game of Thrones, the symbolic moments like this (Sam refusing to be the man his father says he is) can and will be used in the bigger story.
Again, if you have enough wildfire, like in the Blackwater, or a dragon, zombies are not going to be a problem at all. You saw what one flaming ball can do. Although maybe dragon fire can burn White Walkers too, who knows.