I don't disagree with you, but the SCC will likely need to hear from all intervenors directly in order to consider their positions when the ruling is ultimately made.  The fact that they intervened at other court levels probably doesn't matter (to the SCC).  As I understand it it is not atypical that the SCC hears much of the same evidence that was presented to the lower courts (that is of course, along with any new evidence or legal arguments).