Hello Jerome,
Thank you for reaching out! That is correct, in order to be certified through the Advertising Cloud Academy, one must be an Advertising Cloud customer. This is because our in-person certification is a free, added-value option. In the future, we may be exploring an option where one could pay a course fee to attend.
If you're interested in staying up to date on our options, please e-mail the Academy (adacademy@adobe.com) with your contact information so that we may keep you posted.
Please let us know if you have any other questions and we'd be happy to help!