So I'm not your lawyer, I'm not a lawyer at all, and, as with all licensing questions, you need to contact support / your account team to get a definitive answer from your vendor (Elastic).
Any other route (including listening to me) could either mean you're violating a license or you're spending more money than you have to.
How I've seen these go in the past is that my commercial agreement includes a provision indicating that I am licensing a "project" for which I plan to leverage Elasticsearch and that I cannot license parts of a "project" with a different license than other parts of the "project".
By linking my clusters with CCS, it's pretty hard to argue these clusters aren't related to the same "project". So if I was linking my clusters with CCS, I would expect to have to license my clusters at the same level (either basic, gold, platinum, or enterprise).
But your commercial agreement may be different and may use different language, so you will definitely want to contact Elastic support or your account representative for an answer.