They've apparently been working on it for two years. So my guess is the new features or changes in career mode and changes to the UI and design were all being done so all they need to is add the new models to the game. In theory they can make physics changes and keep the 2018 models for testing and then switch to the 2019 cars once they had the data.
The other thing is I think people are jumping to conclusions about F2 in this game. No where in the press release does it say this. All it says is they are giving a car model for use in online based on 2019 regulations that had input from Ross Brawn and Pat Simmons, which can have a custom skin and be used in multiplayer. No idea where people are pulling this F2 stuff from and they should prepare to be disappointed.
Its strange there is no way to contact Liberty Media/Formula 1 for game suggestions but my dream scenario would be that they use a base game engine that they sell to us for a price (one off £30/£40) and then we can buy a classic season or the latest season for £20/$20/€20. I'm sure they would make a huge sum of money doing this, some people would buy their favorite seasons others would be buying many more. Anyway thats my ideal but will never happen!