The version of 61.57 on the FAA website certainly does not say you have to visit the sim every six months. It says you must maintain instrument currency by doing 6 approaches etc within 6 months, and that they may be done in a sim. I suppose if you only ever fly in VMC and don't make a point of practicing then that would mean a visit every 6 months, but why would you let that happen?
As is so often the case at certain traning provider companies, the instructor appears not to have a clue what he is talking about.