Without knowing all of the details, it's hard to say, but it would seem the one receiving the amounts from the U.S., at least on the social security amounts, will be taxed at a lesser rate in Canada.
The change that is being proposed here is simply reinstating the position that had been agreed to and was reflected in the Canada-U.S. tax treaty prior to 1996 and putting people who were in that position prior to 1996 back in the position they were in at that time.