Sure, the results are not deterministic in that 100% of the time the exact prompt returns the exact same result, but you can tune your prompts so that 100% of the time they give you a valid result in the result category you were seeking, and with a specific probability distribution of available choices.
Prompts are functions that can take concrete input and create a probabilistic output that can be automated upon. Especially if you only need to output one token, i.e a number, boolean, word, object reference. And for obvious reasons - the further you forecast out in a sequence the less accurate you will be.
As long as you don't change the underlying model, in a massive model with billions of parameters, there are definitely mechanisms and behaviors to discover that you can reason about.