Two years, actually. 2003 by all appearances was a fluke, because Pittsburgh plummeted from having the #5 offense from having the #22 offense. Falling from #5 to #22 in one year signals problems not related to the OC, all else being constant.
Pittsburgh fans hate Mularkey because he doesn't use a smashmouth offense. Buffalo fans hate Mularkey because he sucks as a Head Coach. We didn't hire Mularkey as a Head Coach, last I checked.
And yes, there is a difference between being a successful Head Coach and a successful Offensive Coordinator, just as there's a difference between being a successful college HC and a successful NFL HC.