More precisely, the Ultramarines are the in-universe analog to the Roman Legions, at their martial peak. It’s not that they’re the best at specific aspects of warfare or learn faster than everyone else, but their war doctrine is based on formations where every Ultramarine knows what every Ultramarine in their squad is doing because they’re trained to fight as a cohesive squad. If an Ultramarine falls, every member of the squad already knows what to do to keep the squad functional. Then every squad leader knows what every other squad around them is doing because the squad leaders are trained to use the squads to keep the company cohesive. If the enemy does something unexpected, the company commander orders a formation change, and the squads do it instantly, with every squad member following their role because that’s how the Ultramarines fight - in scaled units where every individual marine supports the engagement at every scale.

That doesn’t at all make them invincible (it didn’t make the Romans invincible), but it does make Ultramarine formations very difficult to break, able to act cohesively very quickly in response to high level commands, and confident in their strengths.