I might get technology wrong, but i think it internaly trying to adapt poly count on screen to match 1 triangle per 1 pixel on screen, so no mattter how many polygons you have in world, amount on screen would be roughly 1920*1080 same as pixels of your screen. Sort of dynamic adaptation of meshed based on depth, the higher distance - the larger polygons you can have to fit in 1 pixel on screen. Something like this. I meant its common sense - you dont need to draw triangles which are much smaller than 1 pixel.
So yes i guess forest with 1 million of triangles per tree is posible, and i guess it is a goal - have same render time no matter how much stuff in the 3d world.