That's what I thought, but we booked a J round-trip for a colleague HKG-JFK (never banked miles with CX before, just moved to HK), and he put it on his new CX Amex. A while later he gets a SL card in the mail. That's the only personal situation I know, but I recall many other posts with similar stories...people booking J or F class travel, not hitting the SL minimum, and still getting SL if they have the card.

As christep says, the SL card is pretty much a moot point anyway if you're flying J.