Is there some fundamental difference between this feature and the built-in in-cabin air filter that my 2002 Nissan Maxima had? It seems like it's the same exact thing, but maybe the Tesla has a better filter and a more air tight cabin?

I bet my 2011 BMW 335i would probably perform just as well since that cabin is much more air tight. All air passes through a massive filter which I believe contains a normal paper element as well as a charcoal element for odor.

Cars have had cabin filters for a long time now. The primary purpose is to reduce dust an allergies.

I have a hard time believing that the car is air tight enough to prevent actual bioweapon particles from entering after any extended period of time. Of course, the amount of particles required to be harmful plays a big role in the overall effectiveness of this.

Is there some other aspect that I am missing?

[EDIT]: It seems like HEPA specifications are more strict than that of the average cabin filter in terms of particle size. This clearly separates the Telsa from other implementations.