If true that would be one of the great tax shelters of all times. it is not true. you are subject to tax.This is what I understood from the question that you have put.
1. You are trading on behalf of a person who lives somewhere outside Canada, while you living in Canada.
2. The money gets transferred to you is your account is the salary, wages or commission that you earned from trading.
In this case, as I understand, as your income is not generated from any sources within Canada; you are not subject to the income tax.
P.S. It is wise to consult a tax lawyer.