Declarativeness (vs imperativeness) is the property of a language. Whether it allows you to express the desired state, as opposed to a sequence of simple operations.

Idempotency is the runtime behavior.

You can absolutely write idempotent program using imperative language (case in point: chef and its recipe can be expressed as machine code, which is a very imperative language, yet it is idempotent when you run it).

Thanks for this explanation!