Could the article you had in mind be this?
In any case, Eliezer has touched on this point multiple times in the sequences, often as a side note in posts on other topics. (See for example in Why Our Kind Can't Cooperate.) It's an important point, regardless.
No, that wasn't it. I don't think it was by Eliezer. And I think it was a featured or promoted article in Main.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.