Cookies
It describes the site as it works today. CardMath's own pages don't set a cookie and there's no banner.
What the site does keep is session storage in the open tab: a session id, the last card page, the last guide, and the scroll position. If the address includes utm_source, utm_medium, utm_campaign, or utm_content, it stores those too, and only those. Closing the tab clears it. The site also sends the first-party page events described on the privacy page. Those events do not include the amounts you type.