You still don't understand how Kroom or how Law's power works.
Room is just the size of the area that Law's power is limited to. It's the power Law uses inside room that allows him to manipulate people/objects.
Kroom is no different. It's like he's compact his power and can use it more freely.
As we can see in the scene, instead of Law creating a large room. He creates a small room with Kroom that he surrounds his weapon.
The power to manipulate anything in Kroom is the same as Room. The power to pierce Big Mom come from Law manipulating his sword inside Kroom.
I'm not saying haki stops Kroom. I'm saying it stop the power he's using inside Kroom to pierce people.