As someone who participated in the application process—Thank goodness I got the prompt that INCLUDED the extra paragraph! I would not have spent as long on it at all, had I not thought that it was actually being used for something.
My thought process when I was working on it went something like: “Well, it’s a lot of “free” work, but I’m on their volunteer list anyway, and would happily do it, if they sent it out that way.” I was definitely focusing on the “doing something useful for SI, whether I get the job or not,” rather than “put in a whole bunch of effort on a random task the off-chance of getting a job.”
If I had NOT thought that the work was being used for a post or somesuch, then I would have put in maybe a quarter of the time on it, at most, and almost certainly NOT gotten the job.
...Now I feel sort of bad for the people who didn’t get the extra paragraph...