I liked this comment on Alicorn's post: "(pain) makes you want to pull away; it's a flinch, abstracted". What seems to matter about pain, when I think about scenarios such as the one you proposed, is its permanent aversive effect, something not present in simulated pain.

Trying to frame this in terms of anticipated experiences, the question I would want to ask about the posited ECP is, "if I meet this ECP again will they hold it against me that I failed to press the button, because of negative reinforcement in our first encounter". The way you've framed the thought experiment suggests that they won't have a memory of the encounter, in fact that I'm not even likely to think of them as an entity I might "meet".