That is true. But besieging an enemy and being besieged is still a large part of it. In the trailers it shows a lot of siege gameplay and not much other combat, at least that's what I saw when I watched them. I realize that it will actually most likely make up the minority of all combat because you can only siege your opponent so many times before you win or he pushes you back to fighting in the middle-ground. Then, there's where you just skirmish with the enemy's armies and patrols. Then there's naval combat which doesn't include sieging except in a few rare cases. Then there are quests, random monsters that you meet in the wilderness, and fighting against the AI.
Picture made by Aametherar the orc (Elves are the best!)