From the
T&C's.
19.1.5 The travel required in each Membership Year to attain or retain Silver, Gold or Platinum status level must include a minimum of four Segments on Qantas or QantasLink.
I'm sure that if Qantas are going to start enforcing this rule, that these T&C's will need to be altered so that flights on Jetconnect and Australian Airlines would qualify.
I was not so sure about jet:-: however, so I rechecked a copy the email referred to in the OP:
An important note about Status credits
To attain or keep your Silver, Gold or Platinum membership, you need to earn the required number of Status credits plus, you also need to fly at least 4 eligible flights segments on Qantas, QantasLink or Jetstar during your membership year
I think that bit about Jet:-: is new, maybe that's part of the reason for the issue to be raised in the newsletters.