That is the inference.

Then justice is not good for much. But let us consider this further point: Is not he who can best strike a blow in a boxing match or in any kind of fighting best able to ward off a blow?

Certainly.

And he who is most skilful in preventing or escaping from a disease is best able to create one?

True.

And he is the best guard of a camp who is best able to steal a march upon the enemy?

Certainly.

Then he who is a good keeper of anything is also a good thief?

That, I suppose, is to be inferred.

Then if the just man is good at keeping money, he is good at stealing it.