A way companies get away with excessive credit card surcharges

As others have noted, the discount is for being willing to have a direct debit from the types of accounts listed (which does not include credit cards from which direct debits are also possible).
So the extra cost to you of paying by credit card when billed, not via direct debit using an approved method, is 4% pus the credit card surcharge.
Even if the discount had been for paying with a non credit card method, I doubt that the ACCC would consider it a credit card surcharge. This is because when I queried with them a 3% discount being offered for paying by cash I was advised that a discount for paying with another method was not considered to be a credit card surcharge.