Wanting and actually being capable aren't the same thing. It'd be just hubris on Rowan's part, and we know where that leads.
I mean, even the twins aren't dumb enough to presume they don't need any help.
Yes, but that's largely because the twins are mostly inept once you factor out their physical and magical abilities. You have the red one who is, if we're being honest, dumb as a sack of hammers. Yes he's pushed as a martial threat but it's made pretty clear that Andras leadership capabilities are limited once you move him past simple, brutish orcs. Any sort of complexity and he and Jezera let Rowan handle it, which brings us around the Jezera. Jezera is the one who talks, the one who thinks, the one who schemes, but she's also very, very childish, petty, and arrogant. Jezera's childish nature has been shown multiple times, with at least one instance causing her to lose her temper and engage in wanton murder. Without Rowan their plans go one of two ways: either they have to recruit multiple people to pick up his slack or they fumble in obscurity until they run afoul of a larger military force which wipes Castle Bloodmeen off the face of the planet.
One thing that needs to be remembered is that Rowan would be a Gary Stu if not for the presence of Alexia and how bad his strategic position is within the castle. Andras and Jezera could both paste him against the wall if they so choose and having to protect Alexia puts Rowan at a disadvantage at all times. However if you took Alexia out of the situation, find a way to remove her necklace and get her out of the castle, Rowan would immediately be at an advantage. Rowan is an expert combatant with years of military experience under his belt and time spent playing a very important role in the strategy of the war against Karnas.
We know from the fact that twins specifically sought him out that he has a very broad range of skills and a reputation that would be advantageous to their goals. Orcs fear him, the goblin tribes worship his strategic ability, and his time in the war ingratiated him with the nobility. Andras constantly bullies and belittles Rowan because he knows in the right circumstances Rowan could kill him, Jezera treats Rowan like a pet because she knows he could turn their allies against her. Rowan is as dangerous to the twins as he is useful to them.