It's a misunderstanding of how the planned but never implemented Musketeer class (MSK for short) was believed to introduced based on Merlwyb dual wielding flintlock pistols (who is class-wise a MSK) and the MSK class icon being two crossed pistols - which means nothing as other classes that also have dual weapons crossed on their icons... don't actually dual wield (MRD's dual crossed axes, GLA's dual crossed swords).
I still wish MCH was simply an upgrade from MSK, but sadly that train has long since left the station so it's one of those oddities of FFXIV 1.0 that was planned but consigned to oblivion due to it's muddled development and has long since been abandoned by the development team.
