Jack didn't know the Master. Even if this version of the Doctor wasn't the one who'd known this version of him, he had an inbuilt trust of the Doctor, shared and grown between them from the moment the Doctor had looked at him with Rose and seen straight through the smile and the bonhomie to the desperate conman beneath - and still trusted him.
He was still distantly aware of the Master's presence, but his focus was on the Doctor that golden warmth, taking him back forward through the memories.