The same is not true of these models. To my knowledge no company has retrained a model from scratch to make a modification to it. They make new models, but these are fundamentally different works with different parameter counts and architectures. When they want to improve on a model that they already built, they fine tune the weights.
If that's what companies that own all the IP do, that tells me that the weights themselves are the preferred form for making modifications, which makes them source code under the gpl's definition.