Yes it is limited. But I think what's happened here is that it could also be considered common because no matter what you will have a chance to battle and catch Articuno in HG/SS. So since you have a 100% chance of finding it, it is listed as common, unlike a 20% chance of finding a Pokemon in grass. I think they should list is as both limited and common.
The site might want to check the Pokemon with similar situations in HG/SS, such as Moltres, Zapdos, Mewtwo.