Would you say that once a model is trained, there's no need to go back and re-train it, even if you want to, say, remove some material from the training set? Anything can be done just with the weights? That's a big surprise to me.

Of course people hack binaries too, and binaries are obviously not source code. I once edited a book in PDF form because we didn't have the original Word/whatever document. It's not hard but a PDF still isn't considered to be source code for documentation despite that.