I think some confusion here is happening between QF selling a CX flight as opposed to QF being a CX partner.
to me, QF acting as sales agent for the CX flight is not related to QF's partnership with CX and definitely not related to QP access rules for a paid QP member.
First, anyone could sell the SYD-HKG-MNL itin on QF/CX. Sure, the QF website will have a bias towards oneworld partners, but many companies or TA's will have a bias (eg: some agents will get a better comission out of airline X to sell their preferred routings/partners). The only time, imho, that QF acting as the sales agent becomes remotely relevant is if they were selling that CX tag flight as a QF codeshare. In the OP's example this is not the case.
QF's partnership with CX as a oneworld member is only relevant in terms of oneworld tier status (eg: Ruby/Sapphire/Emerald) or codeshare routings. In the case of lounge access, it's about the tier status. a Paid QP member has zero oneworld tier status, so access under oneworld rules is not relevant (which is why your paid QP membership would not get you into, The Pier, for example unless there was a separate agreement between QF and CX to cover that).
to me all the talk of since QF sold the CX flight that means lounge entry should be granted is a furphy. QF will quite happily sell you a UA or VX flight but there's no way in hell a UA lounge will let me in as a QF P1 just because they sold the ticket.
I do absolutely understand the idea/theory that as a QP member seeking access to a QF operated lounge (in HKG) prior to a QF partner flight they should be allowed access but as covered many times the rules say no. I also understand the OP does recognise the rules are there and is not upset by the denial under those rules, but more the logic behind it.