Comment They might not have a leg (Score 2) 54
Taking things literally: GFDL allows the User to use the Original Work for free for any Purpose. Substitute User=Anthropic, Purpose=Training.
The training set, possibly the model weights blob, and maybe even the server that takes API requests and streams the responses back to the clients would be Derived Works. So any User2 who receives them may ask for Corresponding Source.
Problem is, that set of User2 is a singleton, namely, { Anthropic }. Actual users do not receive the weights or the server.