It appears that the charm of riposte does not work with the sword. Since the sword's secondary move has been described as a parry/riposte by the devs, the charm should either be made to work with the sword or descriptions of the sword's ability should be clarified.
Turns out I'm totally wrong. The charm does work with the sword, and I wasn't thorough with my testing. In this case, I ask instead that the charm's description be updated.