Imagine for a moment the Federal Government had not introduced Jobkeeper or any other assistance payment directly linked to stood-down employees; for example an extension of the 'dole' for those not receiving any form of salary or wages.
Stand-downs would still have occurred as specified in various industrial awards and agreements. It's for that same reason that Qantas and probably many other employers, can continue to stand-down staff according to the relative agreement, award or the FW act once Jobkeeper ceases.
As for a trigger to cease stand-downs within Qantas and in particular for it's Longhaul Pilots, that will almost certainly be for a court to decide as I'd be fairly certain that Qantas will wish to extend it as long as possible and the Pilot group will be looking for an expedited return to minimum pay even (if it's at a reduced MGH).