That is a Qantas Platinum benefit and not a OW Emerald benefit -- so you don't get this as an AA EXP, for example. The extra piece is for checked baggage only (the 15kg extra weight is on routes that are not by the piece). There is no formal benefit of extra carry-on, however QF will allow anything onboard in F if you can get it past airport security. Sometimes the carry-ons are weighed at the security checkpoint at some airports (such as SYD), and when that happens the carry-on weight limit is strictly enforced (they are also enforced for Y all the time).