Sigh do you know how kidds awakening work?
He is magnetic. His magnetic field attracts metal since metal is naturally attracted to magnetic force fields. His awakening applies what he does to himself to others.
Can kidd attract objects not metal based to himself? No.
Therefore he can not cast a magnetic forcefully to make others attracted by materials not metal based.
The stones you are referring to literally has METAL inside of it
He has put a „+“ on one object and the „-„ on another. If, like you said, he just magnetized one object (I.e. Big Mum) if wouldn’t be necessary to put the other sign on an object (the tower), since big mum must have pulled the object alone by magnetism, also she wouldn’t have pulled only one object, she would have pulled all metal objects.
But kid created an own separated magnetic field which only worked between the marked objects, no matter what structure these objects are.