"But measured as a percentage of vehicle production in U.S. dollars, electricity rates fell in Ontario in that period"

In US dollars. That is the key point here. 

Their study ran from 2010 to 2015.  In 2010 the Can$ hit a high of $1.02 US. In 2015 it hit a low of about $0.70 US.  Without the drop in the Can$ the picture looks a little different doesn't it?

Depending on how you state it this is the reality.  From 2010 to 2015 the Can$ fell 31%. 

Or you could also say that the Can$ was 46% higher in 2010 than it is today. 

Either way, if it weren't for the foreign exchange difference, the Canadian automakers would not be competitive and rising electricity rates (they didn't drop 31% like the dollar did) are not helping one bit!