Nearly the same is not the same. According to
this story the they expect a 1 mpg improvement with the Hurricane over the Hemi. The $5.50 is for every 1/10th MPG below the standard times number of vehicles sold. So, that 1 mpg improvement can easily save them well over $30 million in fines annually.
Also, it would not surprise me if tougher emission pollution standards coincide with tougher CAFE MPG standards. It's my understanding the Hurricane engine is a much cleaner engine than the Hemi.
Though some here don't believe it, another of the reasons given for the switch is that the Hurricane engine is cheaper to build and maintain. They can offer an HO version that uses most of the same parts as the standard version. I don't know why Stellantis would lie about it being cheaper to build and maintain. It seems reasonable to believe a smaller inline 6 would be cheaper to manufacture even if you threw in a couple of turbos.