The statement is correct, you must wait until the autorities send you the fine. the fine could come from dgt (Spanish traffic department), regional or local authorities.

When you recive it you will have 20 days to pay with 50% discount if you dont want to dispute the fine. You can pay online or with bank tranfer, you will need the expediente number, that will differ from the one that is in the sixt notification.

If you live in EU you will recive it for sure, but outside from EU...