In short, this should not impact the USMCA. I believe the Department of Finance officials mentioned it.
From a U.S. perspective, the U.S. has detailed an objective and “easy to understand whether they apply or not” limitation on benefits rules in their treaties both with Mexico and Canada. For cross-border investments with the U.S., then, it's just an exercise of going through those detailed rules and determining whether treaty benefits apply, and you will know whether they apply or not.
With respect to Canada and Mexico, we're going to have a principal purposes test once this is ratified in both Canada and Mexico. For investments between those countries, then, it's going to be the ambiguous principal purposes test to determine whether treaty benefits apply.
That's separate, as you said, from the USMCA, though, because I think the USMCA just does not impact on whether treaty benefits apply between Canada, the U.S. and Mexico.