RC Master originally submitted a speed run on Heroic difficulty PoA that was much slower than this. He soon realised he could to better and cut nearly a minute off that run, to present you with the 12:08 you see today!
In his own words:
Lower difficulty, more Speed.
Also: must… speed run… get. rid…. of… bad runs… that… i. made! ARGH!!
Halo: Reach The Pillar of Autumn Heroic in 12:08 by RC Master