"Games that have almost identical aesthetics"
I'm not seeing how it's identical. I'm not saying that the games aren't comparable at all (in fact, they're probably related more than most people realize, Playground itself has a lot of former Codemaster's employees and many people have moved between studios), but to say that it's copying Horizon is selling it sort.
I can see elements of the Dirt 2/3/Showdown era in this trailer. The most striking change would be them dropping the GRID-esque color grading in favor for a oversaturated look, but even then it's still noticeably different. Horizon's style is more tame and sterilized.
"dynamic time and weather in relatively speaking"
Wow, that is totally something that's never been done before Horizon.
"open maps"
I see nothing mentioning open maps.
"shown in a trailer that, without knowledge of it being from codemasters or being Dirt, could just as easily have been a trailer for a Horizon game or Expansion for one"
Fair enough. Marketing teams aren't exactly the most original people in the world.