And so does a burning candle. Carbon nanotubes(CNTs) can be formed from just about any reaction involving an excess of carbon. It is the amount and purity involved that is interesting. If you look hard enough you'll find them everywhere.

I would like to know what volume fraction of CNTs and cementite (Fe3C) nanowires were present. If it was a significant fraction they could have contributed to the swords properties by the rule of mixtures.