I think they are absolutely 100% getting kickbacks in some forms. Not necessarily monetary. My basis is that years ago when I was very active in my corners of the internet I got tons of stuff just for knowing the right people. I was far from any sort of power mod. Reddit power mod is much higher profile. So there's no way they aren't getting benefits.

Another thing is I've been on reddit since early on. I lurked a community that was a few hundred subscribers. Today it is over a hundred thousand. I know the familiar usernames and the little cliques of their inner circles. Over the years there have been a few promotional giveaways on the subreddit. Coincidentally the familiar usernames won the prizes. It was rigged. Had to be.

Maybe most reddit mods work for free. The mod of that random little subreddit? Probably working for free. The power users though? There's no doubt in my mind they've been getting compensations. The power users want reddit to continue succeeding to their benefit (whatever it maybe) as much as reddit needs them to keep working.