It's a bit more complex than that. Reddit hires staff to do moderation. If moderation was done solely by users, and never by paid staff, your analogy would hold more water. However, because there is a mix of paid and unpaid labor doing the same tasks, there is enough gray area that a court could weigh in either direction (although I think it is unlikely that one would find for the mods, personally).
A better analogy would be that reddit had a landscaping business, and hired some workers to do landscaping, and you just tagged along and did unpaid work for several years. Sure, the owner did tell you he wasn't ever going to pay you for your work, and you agreed to that. But the owner sold and profited off the labor you provided alongside his paid laborers. He did this knowingly.
There may be a case there.