The context here is bypassing the canal for a railroad across Mexico. While you have described the problem with the Jones act, the premises is we are loading/unloading in Mexico that changes a lot. Smaller ships can go to many different ports - large ones would probably still got to North Carolina, but we can sort in Mexico which means some (likely small) shipping company can run routes to smaller ports, they can take exports from those smaller ports and then sorting in Mexico can get it on/off the right ships. For US east coast to cargo that is crossing the pacific this makes sense (it doesn't make sense for cargo crossing the Atlantic though, so the Jones act is still a problem)