You only need this if you are a researcher. Undergraduate knowledge of Calculus and Linear Algebra is more than enough to have quite a good understanding of ML in general, and LLMs in particular.

Maybe a very small bit of Information Theory (a couple of Shannon's papers are enough) and some classical books on Natural Language Processing from the late 90s and early 2000 so you have an idea of what Language Models are outside the modern Deep Learning driven approach.