It's not a strawman, it's an analogy.

In order to be a man of straw, it would have to claim that AI "just is" a well-poisoning machine (in the literal sense of a machine that inserts poison into a hole in the ground containing H2O).

Now, that alone does not mean that it goes through; the analogy works as an intuition pump, and those can be flakey (see https://en.wikipedia.org/wiki/Intuition_pump)

But nor does it mean that a fallacy has been committed

Let's wait and see. I can see plenty of ways in which that could come true.

Not AI as a whole. Particular activities to do with AI which would generally be deemed not only dangerous, but also not having any clear non-malicious purpose.

So if there was a strawman argument here at all, it's literally the one you just made!