The same as when you get a binary executable that is supposed to be compiled from open source files, inspection will not help you.

If you have the training data and code, you have to recreate the LLM by doing the training yourself, to be sure that the weights are what they are claimed to be.

Having the training data and code would be very nice, but for most people it would not guarantee more security than when you get just the open weights.

The open weights are really usable by anyone, because even the biggest models can be run on a mini-PC or laptop, albeit very slowly (i.e. with weights on SSDs).

On the other hand, with training data of hundreds or thousands of terabytes, most would not even be able to download and store that, much less to rerun the training.

I would very much want to have access to the training data, but because I would want to be able to run traditional search on it, to validate when in doubt what the LLM says, not because I hope to be able to recreate the LLM.