This is how the rule is stated on the J-1 website:
How long before my J-1 program starts can I arrive in the United
States? How long can I remain after my program ends? You may not
arrive more than 30 days before the program start date shown on your
DS-2019. Upon completion of your exchange program, you have a grace
period of 30 days to depart the United States.
I believe that this rule applies specifically for entering the United States using the J-1 visa. But to my understanding this does not preclude you from entering the US under the VWP in advance of using the J-1 as long as (and I think this applies to any visa anywhere) you have sufficient proof that you will be leaving the country to return to Canada. A ticket, tenancy agreement for where you are living, and so forth are the standard things to be carrying.
I had a J-1 visa to do a summer camp in the US, but did some travelling in the US prior to that under the Visa Waiver Program. I then went through Canada and back into the US about a month later, entering on my J-1 visa.
For a concrete answer, I would email your closest US embassy.
So, on the travel.state.gov website, it does say the following:
If you want an earlier entry in the U.S. (more than 30 days prior to
the course start date), you must qualify for, and obtain a visitor
visa, as explained below; however, this is strongly discouraged.
BUT, it then goes on to explain that this is because switching visa status within the US is a lengthy process and you are not allowed to start your J-1 program until it is complete. Since you won't be doing this, I think it is a non-issue.