I know nobody's doubting the veracity of this any more, but it's worth mentioning anyway: The recent online car list lines up pretty well with it.
Each car has a numerical ID, that you can see by the URL when you click into a car (no, they don't leak any unreleased cars before you ask). The final car by ID that was in at launch is the new-gen Nissan Z at 3504. 3505 is skipped, 3506 is the second-gen BRZ from the first update, and every car after that is a new-to-series DLC car that is not in the datamine, with the sole exception of the 400R. The final car is the N-One at 3576.
But if you look at the new-to-series cars that were in the datamine, their IDs are all in the 3400s. The recent 812 Superfast is ID 3470 for example. The earliest one is the Giulia GTAm, all the way down at 3411 - for context, that is a lower number than even some late GT Sport DLC cars like the 2020 Supra and the RX-Vision GT3.
(You can only test this with new-to-series cars and cars returning from GT3 or earlier; returning cars that were in any game from GT4 onwards, such as the Celica rally car or the Evo 8, seemingly reclaim their old ID slots.)