He's talking about steerer tube thickness at the butt. I just measured a 531 steerer I've got laying around. Just about .09" or 2.28 mm. Sticking an Everest crown on brings it to about .21" or a little over 5 mm.
Also FWIW, I don't know too many people who consider SL (.9/.6/.9) to be "thin", unless it's being compared to something sold by Sears. Especially in the era of OS tubing, .9 seems on the thick side to me.
Regardless of the exact measurements, I would have a really hard time believing that the base of the butted portion of the steerer is a failure point. And regardless of all of this latest nitpick, I'd still be interested in pics of a tube that was "can opened" by an unfiled lug. I'm really starting to believe that it's urban legend.