Why can't you improve it or fit it to your data?

This can be cross-compiled/ported in the Linux analogy. The Linux analogy would be more like: a kernel dev wrote code for some part of the Linux kernel using JetBrains' CLion. He used features of CLion that made this process much easer than if he had written the code using `nano`. By your logic, the resulting kernel code is not "open" because the tooling used to create it is not open. This is, of course, nonsense.

I agree that the project as a whole is less open than it could be, but the weights are indeed as open as they can be, no scare quotes required.