Yes, precisely. If fine tuning is the preferred method of modification, then under the GPL the weights are source code.

Personally I think the truth is greyer than black and white.

Fine-tuning is like statically linking a large library (available in binary form only) to a small app. (An optimizing linker can blend app and library code so much they cannot be distinguished.) Weights intended for fine-tuning are still a binary, just a reusable one. Nothing new here.