I am pretty sure that SpeedTree just takes your art assets and uses them to display trees in the best performance and most immersive way possible. It probably has some default art included, but you would normally still need to supply the art (textures, bump maps, etc.). Compare Skyrim LE, Skyrim SE and then say a really well modded version of the game using the latest mods. You'll see an evolution in progress from barely passable to stuff that is truly incredible.

However, this all depends on their artists being any good.