I don't think handling them as one turret would work, unless they were all one subobject, but then they'd have to share one pivot point, so unless they were stacked on the same vertical axis (unlikely on something as flat as the ISD), I don't think that could work. Unless I've grossly misunderstood something about how turrets work. I'm still learning a bit.
Wikipedia seems to agree with the Star Wars Databank, but it was probably the source of that information anyway. I'm not sure what the origin of Wookieepedia's figures is, although I tend to like it a bit more. It doesn't create such a large gap between the Imperial capital ships in terms of armament and makes it easier to scale down the SSD's firepower without making it almost as weak as a ISD. Does anyone know what makes starwars.com 'official' anyway? I mean, who checks and verifies the information there?
Edit: Just checked the SWTC, his talk about the turrets seems to agree more with the Wookieepedia information, and was probably the source of that claim.
Star Destroyer Weaponry