No offense, but we have no canonical evidence to base such an assertion. He is a military commander, and that's all we know from on screen evidence. Leia's position is one of authority, as almost all of the staff defer to her decisions, and her orders, save for Han Solo.Leia is a political "leader", a politician. She is not a political "officer". She is one of the leaders of the Rebel Alliance. Rieekan is not only one of the founders and leaders of the Rebel Alliance, he was definitely one of the Alliance's military leaders and the commander of Echo Base.
The Alliance is a different military organization, and not as organized as contemporary mlitary forces. Leia, by all actions, is de facto leader of Echo Base.