IANAL but as an IP creator I have not heard of "derivative products" in the copyright context. There are "derivative works", which are covered by the same copyright as the original. For example, a translation to another language is a derivative work, a novelisation of a movie, a screen adaptation of a book etc. If some author could have proven that any Anthromic model is a derivative work of theirs then they had the copyright on that model and made mad bucks licensing it back to Anthropic.