But then it's the tools to make the AI that are open source, not the model itself.

I think that we can't have a useful discussion on this if we don't distinguish between the source code of the training framework and the "source code" of the model itself, which is the training data set. E.g, Mistral Nemo can't be considered open source, because there is no Mistral Nemo without the training data set.

It's like with your Doom example - the Doom engine is open source, but Doom itself isn't. Unfortunately, here the analogy falls apart a bit, because there is no logic in the art assets of doom, whereas there is plenty of logic in the dataset for Mistral - enough that the devs said they don't want to disclose it for fear of competition.

This data set logic - incredibly valuable and important for the behavior of the AI, as confirmed by the devs - is why the model is not open source, even though the training framework might be.

Edit:

Another aspect is the spirit of open-source. One of the benefits of OSS is you can study the source code to determine whether the software is in compliance with various regulations - you can audit that software.

How can we audit Mistral Nemo? How can we confirm that it doesn't utilize copyrighted material to provide its answers?