In fact, for mathematics, the M-theory that posits an 11-dimensional space is equally highly abstract and built upon an extremely complex mathematical structure. Whether it is the 11-dimensional space, or the dynamics of strings and branes, or exploring the duality between different theories and the space of branes, as well as non-perturbative techniques...
To fully understand M-theory, one must deeply study those complex mathematics; otherwise, one would barely touch the surface.
This is probably why Edward Witten, though a physicist, was awarded the Fields Medal.
The contributions made in physics were too cutting-edge and advanced to be verified by experiments for the time being, but the contributions to mathematics were evident.
