One can claim 100% recycled cobalt under the mass balance system even if recycled and non-recycled cobalt was mixed as long as the total amount used in production is less or equal to recycled cobalt purchased in the books. At least here[0] they claim their recycled cobalt references are under the mass balance system.

0. https://www.apple.com/newsroom/2023/04/apple-will-use-100-pe...