Sheldon's note at the bottom about many people exceeding those recommendations is quite the understatement. Before rim widths caught up, mountain bikers were blowing the doors off those recs with no issues. Heck, at one point I was using 2.3s with 17mm (ID) rims. That's like 6 boxes into the red on that chart. Heck, I'm STILL two boxes into the red on my MTB and my old Salsa Casseroll that my wife now rides. Zero issues in 23 years. None. Nada. Zip. Would wider rims be better? Sure, for handling. But there is nothing unsafe about it.