I clearly remember a race where a rain started mid-race. Pretty sure I used vanilla version (no third-party fixes).
That still doesn't convince me, sorry. I remember that weather could change. I don't know what they mean with "That way you can race with changable conditions". For example, I found this discussion and no one mentions any apps or plugins for changeable weather, they mention random weather and they all talk as if it works out of the box: https://www.racedepartment.com/threads/understanding-gtr2-weather-change-and-track-surface.213011/
Although I never played GTR2 much, I did drive the Power & Glory mod quite extensively & as I remember the changeable weather setting rarely worked in multiplayer, almost always stayed dry all race.