Doctor Who Season 11 had solid praise, but drew heavy criticism from the fanbase for not opting for villains.
That said, Chibnall really liked subverting expectations last season, so it’s also possible this could all end with another big switcheroo.
Additionally, I don’t believe Chris Chibnall would bring back The Doctor’s greatest foe only for it to be a misdirection that gets reversed in the next episode.
To kick off Season 12 with a fake Master would be quite the slap in the face to those who’ve already been told Season 12 will feature more well-known enemies, and would certainly ruffle a few feathers that don’t need ruffling.