Jodie Whittaker’s post-regenerative scene was the first written for the 2017 Christmas special (and Peter Capaldi’s swansong), Twice Upon A Time!
Ah, deadlines. They’re funny things. Most of us despise them. Douglas Adams loved them – in particular, he said, “I love deadlines. I like the whooshing sound they make as they fly by.” As noted in his excellent Writer’s Tale book, former showrunner, Russell T. Davies often left scripts surprisingly late to submit. We don’t really know what previous showrunner, Steven Moffat did, but he has revealed that the new man-in-charge, Chris Chibnall (Broadchurch) was super-speedy when it came to debuting the Thirteenth Doctor.
Moffat revealed:

“[Chris and I] talked quite early – I think that was actually the first bit of the script written! So he said this is where he wanted Jodie to begin, or the new Doctor to begin, so he sent me those pages and I worked the script up towards it.”

It must be strange for Steven – he’s been in charge of Doctor Who since 2010, but planning his era since before then, and has written for the series since its return in 2005.
Fortunately, he’s more than happy to let Chibnall change what he’s done to the show. At the Radio Times Cover Party, he enthused:

“Doctor Who has to change radically all the time, not because it’s necessary, because it’s fun.”

Does this all mean that Chibnall will be more on-the-ball? Probably not. It’s a writer’s prerogative to sweat over deadlines, and it appears that most stretch things as much as humanly possible. And no, this isn’t some sort of confession. No, not at all.
However, this is pretty indicative of him being enthusiastic to begin his reign over Doctor Who – and who’s to blame him?!
Nonetheless, Doctor Who won’t return to our screens until Autumn 2018.