Ukraine suffer from daily Russia's drone attack. How about a low cost network that detects, disables or shots down those drones before they can get to their targets? That also, and truly is, a defense system.
I don't see anything in YC's call saying they would only fund defensive capabilities (which is itself somewhat of a misnomer since it doesn't take much engineering to turn "defensive" tech into offensive weapons of war). YC looks like it is open to funding all forms of warfare.
Do you have any problems with that?