How do you get land? Through war, specifically by allying with like-minded races. The twins are the rising power, just as Tue-Row is about to be crowned. You think he would stay out of it if the twins ask for an alliance?
Of course all the orcs have heard of the twins, hence they comment on it when Rowan approaches them.
The twins wouldn't suicide into Rastedel. They would have their troops in spies in order before they attack. Which they just did.
You hyperfocus on Rowan because he happens to be the point-of-view MC who is positioned narratively to be at the center of each conflict. But this doesn't conclude that without him, these conflicts wouldn't be solved. They would, just at a slower pace and with someone less ideal.
Oriandu: Total agreement that Rowan is useful and often competent. Doesn't mean the twins couldn't be sufficiently successfull, they have extremely powerful and vast tools, allies and are also able to learn and improve. So I don't get your second paragraph, the twins have builded infrastructure and a very well working base entirely without Rowan.
Obviously, with Rowan present, we can rush through the conquest a bit faster and have the narrative reason that he solves they problem, else we wouldn't have a game. The twins can't be perfect, else Rowan would be superfluous, but this musn't lead into the wrong conclusion that they couldn't succeed without him.