The New Adventures novels suggested regeneration was "improved" for younger Time Persons like Romana ("Newbloods") such that they could choose their appearance and had two hearts from the get-go (the workaround for no-one commenting on Hartnell's single heart was that the second only grew at first regen. Timeless Child buggers that up, but then the same novels had Looming, which is also now retconned out). But the Rani is supposedly the same age as the Doctor, so that wouldn't apply to her.
The explanations for regenerations changed so often between decades... if they treat it seriously and not as a joke, it's not necessarily going to matter as much at this point. At least biregeneration wasn't used as a silly excuse begging far more questions than the Timeless Child had, unless I meant to say that the other way around but "timeless" did seem to try to take itself more seriously.
Besides, in 1966, if the Doctor needed the TARDIS to change appearance like that (hinting at rejuvination), he could potentially live forever, barring accidents... the same argument must have come up after 'The War Games' because the 1970s removed the TARDIS as impetus for the regeneration event, made it biological, added a maximum number of times before their body would die (as long as Rassilon or Borusa or anyone else hands out a new complement of lives, of course), and so on. The modern series sees regeneration not as a morphing effect but as a flaming one, with each new regeneration proving to have destructive abilities within a certain radius as well. Lovely thing, spectacle. Serious in tone those were, but was the underlying idea taken seriously? Just be in the middle of a room full of Daleks, let one Dalek zap you, you regenerate, the energy blows them all up, easy peasy.
The single heart of Hartnell is explained away just as easily as any other telepathic ability that the TARDIS can put out, since it was already causing Ian and Barbara to see or perceive things differently (also remember the melted clock faces) or personality changes (Susan), etc, because the ship was malfunctioning.
But now we have two biregenerations, originally pawned off as a myth but - surprise surprise?- it's happened again, and amazingly there's no song with lyrics like "Oops, we did it again" in the background this time. Yet. But if all of a sudden time lords can split - like mitosis - I'm not expecting anything like gravitas any time soon, noting that the latest use of "mavity" was given quite a different tone this time in the Eurovision episode, for which I'd say "gravitas" except we may as well roll with it and say "mavitas" this time.