Well, open weight is not open source. There's a lot of black box still left in those models.

It's still far more open to investigation and analysis than the likes of ChatGPT, though. Lots of work has been done to allow some degree of investigation into how the "brains" of an LLM work and in the case of Deepseek you can actually apply those techniques to the model you know you're using.

I'm not aware of any realistic way to vet published model weights like that.

It works for some open-source software, because you can download the source and compile it yourself. But most of the "open" models don't publish their entire training set, or enough details on the training for it to be truly reproducible. DeepSeek doesn't publish their data sets, and they probably couldn't, legally, since they "collaborate with third-party data providers to obtain proprietary datasets". And even if they did, it would be prohibitively expensive to train it on your own, unlike simply compiling open-source code.

The actual code for training LLMs is relatively trivial. The secret sauce is the training data and fine-tuning.