"knowledge and experience of nobility" is vastly overselling actual skills (or lack thereof) of most nobles ("and in the game"). When you're surrounded by pampered fools who run at first sight of trouble or get themselves killed in foolish "honorable" charges, merely having good instincts when to run and when to make a stand --or much better, an ambush-- will go a long way towards getting to the top.
Or, put it differently -- the most acclaimed noble of the realm, who has fought side by side with Rowan for years, recognizes his skills as military commander. He doesn't want him promoted to a noble, but that's because it'd defy the holy order where peasants are only ever supposed to be peasants, not because he thinks Rowan lacks what it takes to be a military leader.
And you never thought that Rowan "abilities" and "achievements" were in your words "overselling"... A guy with that curriculum was fool by a pair of brothers who nearly can work together, moreover, this guy, came after the rescue of his wife without a plan nor preparation, his "nobleman comrades" never move a finger for him, a thing is fighting together and another side by side, if you command in different fronts you fight side by side but not exactly together, Rowan could gain charisma between the soldiers because many of them came from the same source as him, from the commoners, and after his "victories" he obtain the sympathy of the army and the acknowledge of the nobles but doesn't mean that he is better than the rest of the entire world, he's on the same level, there are things in the story told of what Rowan has done that doesn't make sense, and we can assume this holes in the narrative or suspicious since the beginning, when Rowan came to live in the Town as others commoners did and after his marriage, why he should be so uncaring about his abilities? in-game in a conversation with the Elder there's an option to tell to the Elder "The Price of Peace is Eternal Vigilance", if Rowan can have that thought is doesn't make any sense that his overall status are just shit, nor less that he had barely a sword but no armor at sight, and without forgetting that in a War against the Father of the Twins there must have been magic casters, how the hell Rowan wouldn't have even basic equipment to resist some magic?, i'm not stating that he could just break the magic cast on him to enslave him but something to be protected against sensorial alterations, fire, frost or some shit like that that could be used in combat. Instead of any of that, he could barely have a sword and one that doesn't look too good...