Many SQ A380 crews came from the jumbo.
Quite possible, also quite possible that they had also flown the A340 beforehand, (the trainers almost certainly did), most, if not all, would have flown the A310. Quite possible that Airbus waived their rule for SQ or that SQ didn't take the "Free Training" offer and simply bought training time.
I believe the Airbus 'Must Have Previous Airbus Experience' requirement applied if they were providing the initial training as a part of the purchase package.