Unless your benchmark for flat plane sound is a Ferrari. The 12C has exactly the sound a flat plane makes. Smooth. No rumble.Learn something new everyday. It definitely doesnt have a flat plane sound, so its surprising to learn that.
American V8, AMG and pretty any other road engines has cross plane so they have that rumbling burbling sound from uneven firing order.
Ferrari just tuned their muffler for a high pitch squeal, that's all.