What she really means:
Why did CULLEN RUTHERFORD, who was the Templar Knight Captain (second in command to Meredith Stannard of all people) become the Inquisition's military leader and strategist when Cassandra could have picked literal army commander AVELINE VALLEN??? She's the obvious choice because of her experience (more integrity & longer time in a higher ranking position commanding more people for a better cause). Not to mention that her favorite series is Swords and Shields which is based on Aveline's life, there is no way in hell she would prefer Cullen over Avvie.
Logistically speaking, she's the CAPTAIN OF THE CITY GUARD in Kirkwall, which is 1) Kirkwall's literal army, and 2) likely far larger than the Templar order in Kirkwall due to the depth and breadth of their duties being far more encompassing. From what Varric said of her, Aveline is not only the clear choice because she's more qualified (she outranks Cullen and has had more experience being the commander of a larger number of people in a job that again, consists of more responsibilities), Varric has told Cassandra about the fantastic positive changes she's made and illustrated how she is an honest person with integrity (literally was promoted because she exposed corruption and has a clear and traceable clean up of corruption from within the Guard), she's publicly available, clearly can handle a number of duties (balancing vigilante work with Hawke, her guard duties, her personal life, and managing the Guard). She is the OBVIOUS CHOICE. Cullen, on the other hand, is in favor of mass murder and if you don't agree with him, MURDERS THREE PEOPLE IN COLD BLOOD (Broken Circle in DAO//end title cards in DAO). He then goes to Kirkwall and learns nothing from his experience, as best evidenced by the fact that he BECOMES MEREDITH STANNARD'S NUMBER 2. BIG RED FLAG (or statue).
And!! He is a templar!!! His fighting style is adapted to fighting mages! While that is important since mages are now more likely to appear since the circles disbanded, most of the fighters who join the Inquisition need to learn skills he can't teach (far less of his training was devoted to non-magical attackers; yes templars are trained sword and shield warriors, but they expend so much time and energy covering magic that they don't spend nearly as much time focused on non-magical combat). You know who could work with non-templar recruits? AVELINE MOTHERFUCKING VALLEN. That's her JOB. SHE ALREADY COMMANDS AN ARMY.She also, y'know, values mage lives (as opposed to Cullen who literally tells you that "mages aren't people like you and me").
Cullen is a shitbag who dehumanizes mages and advocates for their death. Aveline is one of the most level-headed, moral people in the series, second to only Cassandra herself (I don't think anyone has ever called Cass "level-headed" in her life, but she has integrity and is an excellent decision maker, fighter and tactician).