I agree, it should be available to 90D.
For the Model-S, the spoiler is for performance enhancement and make sense for P model only. But for the Model-X that spoiler is adding 1.6% range improvements.
BUT, maybe the 90D have a fixed spoiler. It's my guess since I cannot believe they would impact the range on their "range optimal" model.
We definitively need more information
Another weird thing is my 90D pic shown in design studio has the active spoiler. But I just assume it's a small bug.