So... the Ruth, the Fugitive Doctor, is indeed a past Doctor -- and it ties into the Doctors seen in "The Brain of Morbius." *spoilers&
"The Timeless Children" reveals the birth of the Time Lords and the secret origin of the Doctor. The Doctor is not a Time Lord. The Doctor is the first Time Lord. The Doctor was an abandoned black girl found on a distant planet by a Gallifreyan astronaut, Tecteun. Tecteun adopted the child as her own; the child died in an accident only to regenerate into a new body -- and Tecteun became obsessed with experimenting on the child to learn the secret of regeneration to ward off death. Tecteun forced the child to regenerate multiple times; the girl became a boy, changed race, changed ages -- and while Tecteun couldn't learn anything from the child about where she'd come from her who she was, Tecteun finally learned how to graft regeneration into her own body and then other Gallifreyans but set a limit of 13 lives to control its use.
It's implied that the secret of regeneration also led to time travel, producing the Time Lords while the Time Lords continued experimenting on the child, allowing her to age to adulthood, dispatching her on various missions that would often take lifetimes, wiping her memory each time -- and then, for reasons unknown and erased, the child was mind-wiped once more, regenerated into a male form, locked to 13 lives and starting as the William Hartnell Doctor.
At one point, the Jodie Doctor looks at her own memories and there are clips from previous episodes and one of the clips is the additional faces in "The Brain of Morbius" and another is the Ruth Doctor.
The episode itself is a bit pathetic: Jodie Whittaker spends nearly the whole episode locked in a cell receiving all this exposition. At the end, Chibnall puts her in a position to blow up Gallifrey and the Master and she decides to do it but then hesitates and then stops and then a guest character blows up the planet for her so that the Doctor can run away. Chibnall doesn't have the imagination to come up with a satisfactory solution, so he creates a suicide plan for the Doctor and then has someone else carry it out. He's not a good screenwriter -- but this revelation -- I like how it makes the Doctor even more of a mystery. Who was she before she became the first Time Lord? Why was she abandoned? Where were her parents? Why couldn't she answer any questions about where she came from? Where did she come from?
Why did the Fugitive Doctor's TARDIS look like a police telephone box when the First Doctor's TARDIS only took on that form when landing in 1963 in the series premiere?
It suggests a much lengthier history behind the Doctor and the TARDIS and for every question of the Doctor's origin that's answered, a new question has been raised, and if the Doctor's Time Lord heritage and Gallifreyan origins are no longer where she started, merely a middle ground of her life, then Chibnall's decision to remove the Time Lords and the planet after Steven Moffat restored them makes sense.
It also explains a lot of peculiar continuity errors over the years. The First Doctor said that he had been "exiled" from his home planet, but the Second Doctor later said that he had run away. The Third Doctor said that he had lived for "thousands" of years but the Fourth Doctor confirmed his age to be around 400 years and mentioned that his departure from Gallifrey had involved some scandal he'd fled (which reduces the distinction between exile and running away). There's also those Morbius faces)
The Sixth Doctor was constantly bragging about his experience, but the Seventh Doctor hinted that he had personally interacted with Rassilon and Omega, the founders of Gallifreyan society at the dawn of time which if the Sixth had remembered, he would have discussed repeatedly.
Then there's the confused memories: Seven says he doesn't remember ever being a child while Eight recalls lying in the grass with his father and Ten speaks of running across fields as a boy. It looks like the Doctor's memories were edited to remove her pre-Hartnell lives from mind, but each regeneration seems to have opened up more and more with the Third remembering "thousands of years," the fourth recalling the pre-Hartnell lives -- and even the Second Doctor starting to feel that leaving Gallifrey was due to a subconscious instinct that she had been exploited, experimented upon, abused (and a woman forced to live in a man's body) -- and that her supposed excommunication was in truth an escape.
That said, Chibnall's grasp of plot, action, situation, resolution and exposition remain shockingly poor. I'm not sure what this person is doing writing television. He's a great producer: the anamorphic lens filming, locations, lighting, blocking, interiors and effects are beautiful as is the music -- to the point where I may rewatch Series 11 and 12 with the sound off and the score playing and then fill in the stories myself.
What's most shocking is that he made his Ruth Doctor far more interesting as a character than his ground-up creation starting in series 11. Is he genuinely capable but slagging it all deliberately? That doesn't make sense either.
But he did demystify the show and there's no way to say he reintroduced mystery. Not with the level of crudity he's put in (oh look, someone whose name is in key with the opening theme music caused mass murder of numerous children to a being from a magical other universe where they probably ride on unicorns too, and just to reverse engineer some DNA in a radical form of theft.) The grossest horror gross-out movies are tamer than Teletubbies by comparison to Chibnall's handiwork. Unless it's a lie made by the Master. But Chibnall claimed he wrote this plot back in high school. Yup, it's fanfic at its finest...