I think it's a problem of communication more than about time.
So, I would have wrote them a letter like this:
Hey, admin. I'm actually working on this project already. However, I'm working at my own pace. My hands are tied at the moment and unless I'm compensated I can't dedicated a lot of time to it and your request certainly has implications of time commitment.
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.