You've been reading Stardestroyer.net again, that argument's been conclusively fried here before. (Hell, the article sd.net quotes for support says something different itself.)
Star Destroyer main battery broadside power output is in the kiloton range. How do we know this?
Watch Empire Strikes Back. Asteroids. 'Nuff said. Given the movies the engagement range is pathetically short, only a few dozen kilometers at best. The only other sources, the games and books, seem to agree with this in general. While they have proved able to strike from orbit effectively, this does not require a range much in excess of, say, 100km.
The Covenant have ships capable of engaging at ranges of 350 kilometers on in. Ref. Fall of Reach, among others. There was another ship capable of much more distant engagment ranges mentioned in the same book. (It's worthwhile to remember that as far as canoncity values goes, the Extended Universe for SW loses out to the movies while the books stand equal to the games in regards to Halo. Bungie are greater canon-nazis then any other creator group ever, and I must admit an admiration for that.)