The genericized fluff, not the Cormyr-specific stuff.
There's no requirement a Warlord be a Knight, for one thing, nor did Warlord builds necessarily lead in the literal sense (that's not what the 'Leader' Role meant), while a Bannerette is just like any other archaic military rank in having some formal authority.
What exactly in the Cormyr-specific fluff cannot be applied to the PDK as presented in the SCAG?
Yes, a Warlord doesn't need to be a knight. Which is why the stuff in the SCAG doesn't preclude a Warlord option later on. But a Bennerette is amilitary rank yes, but one given specifically to Knights who were given the right to lead men. In that way a Bennerette and a PDK (Who were commanders in the Cormyrian army) fit together perfectly IMO.