even with t=0 they are stochastic. e.g., non associative nature of floating point operations

That is an artifact of implementation. You can absolutely implement it using strict FP. But even if not, any given implementation will still do things in a specific order which can be documented. And then if you're running quantized (including KV cache), there's a lot less floating point involved.

Doesn’t changing even one word in your prompt affect the output?

Yes, and completely unpredictably.