I'm only going off the bottle which clearly states exactly what you've quoted....except due to the phospherous levels the oil isn't actually an API SN grade, it's an API SL I believe. In effect, the oil meets all requirements for an SN oil but the phospherous levels only meet an API SL. As a result, if you use this oil and the cats stuff up, I'll bet my left one that if Holden knew you were using that oil, they'd reject a warranty claim on the basis that the oil is too high in phospherous levels. In other words, Holden will use any excuse to get out of warranty.
As far as Dexos grades, I only spoke with Castrol tech line the other week and they confirmed that they have no intention of paying for the rights to be Dexos approved. Further, Castrol Australia currently has no oils that meet Dexos spec anyway. I'm only going off their own tech line info and my experiences with Holden warranty. In relation to a $70 oil not stuffing up cats....take Royal Purple for example. That stuff is great, arguably one of the best lubs on the market, but at present is only licensed as an API SL grade oil. Although Royal Purple will tell you they've never had an adverse reaction with cats, if you were the unlucky customer who's cats did blow up (and Holden required you to prove what oil you've been using for warranty purposes)....they may very well deny the warranty because of the API SL grade oil.