Sounds like one of them is wrong, doesn't it?
Seriously, the proof is in the pudding. Over the last 20 years I've done countless action jobs on revolvers with both MIM and case-hardened parts (these days the vast majority are MIM parts), and I'm here to tell you they slick up just fine with ceramic stones. My own main competition revolver, with which I've won lots of matches and made my way to Top-3 finishes at the USPSA Revolver Nationals in 2006, 2007, and 2008, is a plain old 625 with its original MIM guts, which of course I stoned nice and smooth when I put it into competition service in late 2004.