To some people, a supercar has to be unique, that includes the engine.
McLaren F1 has an engine made by BMW M, but it's a unique engine that no one else uses.
MP4-12C also has an engine by an outsider, but it's still a unique design.
Enzo has a engine that at the time of production it wasn't shared with any other car.
Aventador's V12 is also unique.
Our beloved 911's flat 6 is limited to 911s only.
The Ferrari people threw a rage about the California not a true Ferrari because the engine was from Maserati, even though it's a sister company.
It's costly to make a specific engine for a limited production car, but that's THE POINT of having a supercar, uniqueness.
If the engine has to be shared with some other cars, especially a different brand of cars, then that supercar has lost quite a bit of the identity.