That argument is mostly used against the misconception of AI-stans that LLMs are compilers, English is a programming language, and that prompting is a programing. There are many reasons for why that is false, and LLMs being stochastic is one of the more obvious reasons.

But there are more probabilistic elements about LLMs. Inference is by default deterministic for any given input and weights, however it is made stochastic algorithmically, when the model chooses the second or third likeliest outcome with some probability. Training is stochastic, the adjustment of the weights is probabilistic, etc. etc.

You seem to make the fact that our brains are also probabilistic as some sort of a gotcha. However that has never been in dispute. Neurons fire with certain probability, this has been known since the advent of neuroscience. However this is why computers are preferred for certain tasks over human brains. When you run a program you can be certain it behaves in a certain way given a particular input and parameters.