An example might help show what he means, as follows:
Let's say the total cruise price is 5000, and the TA offers a 10% discount in the form of a rebate off the cruise price or a cheque sent to you. If a rebate is chosen you will pay the TA 4500; If you go with a cheque, you will pay the TA 5000 and later a cheque for 500 will be sent to you.
In both cases your net cost would be 4500, described as 'a wash' because the end result is the same, albeit arrived at by different means.
In relation to the cc 3% cashback, if you pay 5000 by cc then you get 150 back and if you pay4500 then you would get 135 back, although he appears to not describe this element accurately!