Didn't Onimaru collect all the swords from the fallen Samurai in preparation for the War?
Ushimaru isn't wielding Nidai Kitetsu in this chapter, but it is possible he left the Sword with Hitetsu knowing he was going to die, so it can be passed down to another, or Onimaru brought it to Hitetsu, as he should know Hitetsu is the one who forged it.