Seriously? There is a difference in intensity between 2 marathons over 17 hours, vs 2 marathons over 8 hours. The higher intensity is harder, that's why you get a medal for finishing fast.
Both of you are kind of right. It's still an insane feat of endurance but averaging an 18 minute mile is pretty damn slow and likely zone 2-3 even in rugged terrain for an athlete like her.
High intensity isn't just a generalized term but specific to heart rate, lactate buildup, and anaerobic states.