Believe whatever you want. I'll stick to the facts.
Ok, the facts so far which seem irrefutable.
1. David Tennant is playing the Doctor.
2. While he's dressed like the Doctor he's wearing an outfit we've never seen before.
3. He's using Whittaker's Tardis
4. He's interacting with Donna and as far as we know the last time he saw Donna he was keeping his distance at her wedding lest he make her head explode and this was at the tail end of his grand farewell tour seemingly minutes before he regenerated.
5. It's definitely set now given Wilf looks older and Donna has a daughter.
So hypothetically what's happening?
1. Somehow Ten found a way to reverse the Doctor Donna issue during his farewell tour but, rather than reversing the damage he instead hopped into the future-changing outfits and Tardis for some reason- so he could have another adventure with Donna who's now able to remember him. At the end of their adventure he went back- changing outfits and Tardis once more- so he could visit her wedding and the regenerate.
2. This isn't Ten, it's the Metracrisis Doctor who's somehow returned from Pete's World having worked out how to save Donna. In returning to our universe he's found a Tardis which just happens to look like 13's.
4. It's all happening in someone's head/ it's an illusion/Land of Fiction/ The Matrix etc.
5. Jodie regenerates into David who'll then regenerate into Nucuti.
I mean Occam's razor and all that.
We do need to find some way of rationalising Doctor numbering. What makes most sense is the idea of an official Doctor. The current long term Doctor, not a one off. In this respect Gatwa is still the 14th Doctor (and Eccleston is still the Ninth because Hurt was a one off Doctor). This means Hurt, the Metacrisis Doctor, Jo Martin's Doctor and, potentially, Tennant 2.0, are still Doctors but they're outside the numbering scheme.
That works quite elegantly except insofar as you have to treat McGann as an official long term Doctor, not a one off Doctor. I mean when McGann was hired it wasn't with a short term intention. He was hired to be
The Doctor, so that does make him different to Hurt/Martin/Tennant 2.0 who were only ever hired as short term options.
I may have given this too much thought.