I have never seen any such rule stated in OneWorld Explorer, Global Explorer or Circle Pacific rules. In fact, on my last DONE4 in July/August I had several QF codeshares on AA and BA operated flights. And on my DCIR22 ticketed yesterday for departure next week includes a QF codeshare on AA metal.
As far as I can tell, the OneWorld Explorer fare rules were last updated on 24th August, and the change at that time was that effective immediately AA ticketing restriction with regards to Lybia removed from para. 15 (sales restrictions); D RBD added for LP internation flights.
Ask the agent to read fare rules section 4 (Flight Application/Routings), subsection G, which states:
(G) TRAVEL ON ANY AA/AY/BA/CX/EI/IB/LA/QF/LP CODESHARE SERVICE OPERATED BY AA/AY/BA/CX/EI/IB/LA/QF/ LP IS PERMITTED. OTHER CODESHARE SERVICES NOT PERMITTED.
EXCEPTION TRAVEL NOT PERMITTED ON CX FLIGHTS BETWEEN HONG KONG AND VIETNAM