Skimming over the paper it seems their method of translating factorization to the optimization problem consists of simplifying equations without justification that this can be done efficiently. I suspect that their preprocessing step is already NP-hard.
The second and more important issue, is that the overall strategy---of translating a problem with a sub-exponential classical complexity (via the Number Field Sieve) to an optimization problem with exponential runtime---is not expected to succeed, as confirmed by careful measurements in our paper.