Multiple studies demonstrate that scattering amplitudes and the S-matrix in N=4 Super-Yang-Mills theory can be explicitly calculated using dualities, amplituhedron geometry, and integrability.
The retrieved literature contains multiple papers specifically dedicated to calculating scattering amplitudes, form factors, and analyzing the geometric structures (like the amplituhedron) of N=4 Super-Yang-Mills theory. Papers [2], [5], and [11] all directly examine and compute these quantities, thus supporting the claim that the S-matrix is calculable in this theory. No papers refute the claim.