Sapphires are allowed access to any oneworld Business lounge, regardless of class of travel. So you can "lounge shop" around the network however you please, as long as you see the Sapphire gemstone outside of the lounge.

The exception you mention is only applicable to people who pay for an annual Admirals Club (or Qantas Club) membership and do not hold a top tier status with a oneworld member. Those people would only be entitled to access at those AA/QF lounges as stipulated in those programmes, and would not be entitled to access, say, a CX lounge.