2 years in dev for a racing game is a little long unless the engine used is different.
Studio 397's purchase of the code/software for rF2 might be just to make things 'official' in the long run.
ISI based titles have been ok for stockcar mods (rF) or even striped down versions like ARCA Sim Racing ('08).
If they've got a platform with variable weather working for stockcars... then the changing wind would effect your car. Cloud cover would effect the grip and cooling temps of the night would effect the handling.
It's a little unclear what exactly the new NASCAR game will consist of. The only thing we know for sure (as of the TraxionGG interview from last week) is that Studio 397 has been building the physics in it using RF2's code base, and it's been in development for about 2 years.
Where I'm less clear is whether 704 is still involved (I assume they are) or to what degree. They might be handling the career mode, graphics, sound, etc while 397 handles physics and engine-level stuff.
It's assumed they ARE moving away from Unity as the "host" engine but not necessarily. RF2 physics could certainly have been transplanted into that engine, or they could be building it from the ground up within RF2's existing platform.
No matter how you slice it though, I'm very keen on seeing the fruits of their labor in a few months.