If I understand you correctly, you have misread the Cathay page. It states:
For flights using the baggage allowance 'piece' system: in international itineraries, up to a maximum of two checked bags is allowed – one additional bag (up to 23kg) is applicable when the ticketed allowance is one bag, or one checked bag (up to 23kg) is applicable when there is no ticketed allowance; in purely domestic itineraries, one checked bag (up to 23kg) is applicable when the regular ticketed allowance has no free allowance.
It seems to me that your fare included 2 pieces already, in which case you are at the maximum. Sapphire status does not add a third piece. The oneworld site has wording with the same meaning.