To be clear: within a Marxist framework, the only way to gain capital is to exploit others. So your second half is correct, but your first half is wrong, unless (which I suspect you are) that you're using a different definition of exploitation than the article.

This is correct. Marxist 'exploitation' does not happen until someone profits, to use other people's terminology. This can only happen when someone is in control of the well, and then uses it in some form of economic exchange. A bunch of people building and then using a well does not have this character, at least as far as this particular thought experiment goes.