That's interesting and I hadn't noticed it in the FAQs before, but it's contradicted by the lounge access pages which all include the 'next onward flight' provision:
[TABLE="class: basic"]
[TR="class: alt"]
[TH]Platinum Frequent Flyer
[/TH]
[TD]Next onward flight must be on a Qantas^, Emirates^, Jetstar Airlines+ or
oneworld flight number.#
[/TD]
[/TR]
[/TABLE]
Lounge Locations | Singapore Changi International Airport | The Qantas Singapore Lounge
And this from the QF website at the time of ATA removal:
+ From 1 February 2011, to access Qantas Domestic lounges, Platinum Members must be travelling on any flight the same day that shows Qantas (QF), a Jetstar Airline (Jetstar (JQ), Jetstar Asia (3K) or Valuair (VF)) or a oneworld® flight number on your ticket. To access Qantas International lounges, a Platinum Member's next onward flight that day must show a Qantas (QF), a Jetstar Airline (Jetstar (JQ), Jetstar Asia (3K), Valuair (VF)) or a oneworld® flight number on your ticket. Other lounge access conditions apply