Thinking back, the film created a major continuity error, if I have the chronology correct.
If I remember correctly, as the battle was raging on Scarif, the admiral's fleet was on its way. His ship contained the Tantive IV. Soon afterwards, the star fighters are launched to assist the admiral's fleet. As the fighters leave, we see C-3PO and R2-D2 on the planet. When did the Tantive IV have the time to pick up the droids? And, if they did return to Yavin 4, why did they not drop off the plans to the Rebel Alliance Council?
I don't remember the exact line but on my second viewing I watched with this in mind. There's enough looseness in Mon Mothma's reference to the Admiral to fit the Tantive IV getting up there with the droids before he actually leaves.