As most KDB code is proprietary, one wonders if there's even enough available code to train/steal from.

KDB code is as proprietary as java, python, C++ code found at other financial institutions. What's proprietary is the Q language. not the code you write with it

I think what they mean is that there is less publicly available kdb code for llms to be trained on

No "public" LLM can write KDB yet.