Thanks for the chapter!
Translations a little off. Shirasaya translates to White Sheath. However, the scabbard is black colored or was it suppose to be white and artist made a mistake? In any case, when the demon says Shirasaya he's referring to the scabbard. So MC used his scabbard to make that uppercut on the demon's chin to create an opening long enough to attack with the sword. Although some swordsman can use their scabbard in a two swords style it usually requires a specially made scabbard. Otherwise the scabbard will bend or break after even one attack or defense and more likely get chopped in half if it goes against a blade.
In any case, demon seems to be OP for the MC to fight. Also, that room probably has a lot of other tricks considering he was able to keep the 11 adventurers in it alive and reach in to pull them out of what appeared to be a solid surface. Probably find out after he pulls himself together again as physical wounds seem to be more annoying than deadly considering he was able to grab and reattach his arm in a couple seconds. Guy probably has done a lot of self-experimentation on himself. Which is a bad idea considering what happened to Dr. Jekyll and Mindbender (from G.I. Joe) whose results were very negative.