Only true if you don't consider quarry diving "real" diving. Can you die in a quarry from a diving related problem? yes. Can you die in a simulator from a flying related problem (like, crashing, running out of gas, fire from an engine, or in the recent case, a bolt piercing a fuel tank in a wing)? Nope. (you can die ina simulator, but only if you got mugged or something.) Therefore, a simulator is not flying. Since quarry diving is diving, I think the other suggested analogy, of a pilot having experience only in, say, a flight from Gainesville to Atlanta and back, is more reasonable.
And that pilot understands the basics of flying, and if they flew over mountain ranges or long stretches of water, would probably do ok, but not neceessarily. For example, many of the other pilots in my dad's corportation (he owns a Piper Warrior II with about 5 other guys and they all rent it from the corportation, so he can own a plane without being super rich. It's from the 70's, I believe, not new by any means) cannot function as well as my dad can. He'll go flying with them, and they don't get flight plans organized right and misplace maps and charts of taxiways so when things get tough and they are trying to shoot an ILS with foggles on, they are much more at risk of crashing or hurting themselves for making silly errors that they wouldn't make in a VFR environment. So if they always fly VFR, and suddnely are flying IFR, even if they have an IFR rating and have a few IFR flights and landings under their belts, they aren't as safe.
So the DM here might be like that, he's got his "IFR" training but hasn't done that many IFR flights.
But, that analogy could fall apart. Maybe his quarry has bad conditions, so it's more like he's great at landing but only with a crosswind from one side, and his landing is much worse with a crosswind from the other side. So the DM is used to dealing with low viz, but isn't used to currents at all.
That's where the danger comes in.
Sorry to make such a rambling post, I'm trying to study psychology and do some economics homework too. RJP, you made a good point to counter my good point, and while I think you pointed out a flaw, you didn't correct it perfectly. I think that the other guy corrected it better, and hopefully I have improved on it somewhat too. The more I think about it, the more I think that maybe the DM definitely wasn't as "safe" diving in an unusual environment. Still, I do think he was much safer than someone who'd never dov that environment and the simple fact that he's done ots of diving would prevent him from making a stupid mistake, like running out of air, even if he does make other mistakes (like getting carried away by the current, or peeing in his wetsuit or something horrible like that)