I do agree with your overall sentiment, there isn't much proof that describes that the other planforms can absolutely not provide similar radar reductions across the spectrum if the 3d shape was designed correctly. However Tomboy's conclusions are not unwarranted, the US is and has been leading the pack in term of stealth technologies and is known to be rigorous with it's requirements. With the B-21 Raider being the USAF premier air penetration bomber I doubt they would simply put ease above performance. With research spanning over 4 decades yet the B-21 retains the same general planform as the original B-2, it's hard to overlook that in the basic 2d polar plot, layout D's planform showcases the best performance, while in practice the world leader in stealth technology uses Layout D time and time again.