Reddit is a site that lets you create your own community about whatever (within some pretty permissive ToS grounds) you want and run it however you want. Why would you get paid for running your own little pet project community? I don't see how it's any more relevant than the labor of all the people posting and voting on content.
To be fair it does seem like the group behind this study is also interested in the latter too. I'm just not sure I agree with the premise any more than I would agree with the premise that people calling each other on the phone are providing free labor to the phone company since you normally have to pay professionals (eg radio hosts) to produce audio content.