I'd like to assume that we all know that gameplay is in no way canonical, and as such no gameplay mode does truly represent the realities of the Haloverse. At least, in terms of weapons, resilience and enemy/allied AI. What gameplay probably does represent accurately however, would be the level of difficulty you face in general, enemy and allied numbers, and how well you, your allies and the enemy AI units match up toghether in terms of combat proficiency. As such, it's said that Heroic is the best to emulate this.
Of course, a big part of this is assumptions and guesses.