I think I’m going to side with
@Wake2skate on this one and say it’s not the 918 Spyder. If you look at the silhouette in Kaz’s Twitter post, you’ll notice that there is a crease that shows unevenness at the bonnet/frunk at the bottom-left car:
View attachment 1175885
This indicates that the light (presumably coming from one source only) has been reflected in various directions at that area, suggesting that maybe there is a panel line there, or a hump nearer the center of the car compared to the sides. If you compare this to a random photo I found of the 918 from
this source (courtesy of Google):
View attachment 1175887
You’ll find that the real-life 918’s frunk is simply too smooth for the crease to appear.
I don’t know which car it is, but I doubt it’s the 918 Spyder, at least not in this road-car form.