Doctor Who actress Jo Martin insists she's not the new Doctor. Doctor Who season 12 delivered on its promise to completely transform the show's history. At its core was the shocking revelation the Doctor is the Timeless Child, a mysterious being who emerged from a Boundary to another dimension or universe.
In addition, the episode "Fugitive of the Judoon" introduced viewers - and, indeed, Jodie Whittaker's Doctor herself - to a never-before-seen incarnation of the Doctor, played by Martin. The episode itself strongly seemed to hint she was pre-Whittaker, and indeed pre-William Hartnell as well. Her TARDIS interior bears a close resemblance to Hartnell's, she has Hartnell's habit of referring to the TARDIS as her "ship," and she doesn't know what the sonic screwdriver is. That last piece of evidence is particularly significant, because the sonic screwdriver was invented by Patrick Troughton's "Second" Doctor.
Martin has obliquely referenced this in an interview in Doctor Who Magazine #549, where she recalls being approached by friends and family after the episode aired, amused at their reactions. As she explained it:
The thing is, not everyone saw the episode when it went out, but suddenly they saw all the publicity that blew up around it and read it in the papers or saw it on social media. It meant they didn't get to see the reveal first-hand, which, for me, was the best part. They were like, "You're the new Doctor!" And I'm like, "Well... I'm not the new Doctor. I'm a Doctor. You'll have to watch the episode."
Some fans had argued Martin couldn't possibly be a previous Doctor, and that instead she originated from the future, perhaps after experiencing another mind-wipe of some kind - explaining why she didn't remember Whittaker's Doctor or the sonic screwdriver. It seems Martin herself doesn't share this view, instead definitely considering her Doctor to be one from the past. She doesn't definitively indicate her Doctor is pre-Hartnell, but that currently seems the most likely implication. There is one anomaly, however - her TARDIS has already been trapped in the shape of a Police Box, and that was supposed to only happen in the Hartnell era.
This transforms Martin's Doctor into a strong piece of evidence supporting Doctor Who season 12's Timeless Child twist. Time Lords can only regenerate 12 times, and the Doctor apparently ended his natural regeneration cycle at the end of the Matt Smith era, meaning there's no room for Martin. The Doctor Who season finale, "The Timeless Children," saw the Master reveal the Doctor was not a Time Lord at all, but was actually the Timeless Child - a being from another dimension or universe, gifted with unlimited regenerations, and the base genetic code of the entire Time Lord race. Some fans have argued the Master could be lying, but the existence of another Doctor supports this. What's more, the Master used the Matrix to present the Timeless Child recruited by a secretive Time Lord agency called the Division - and they could well be the ones Martin's Doctor was running from.
More: Doctor Who's Timeless Child Reveal Breaks Matt Smith's Name & Regeneration
Source: Doctor Who Magazine #549