If it was average per moderator, I'm pretty sure the article would attracted more eyeballs with the headline $3.4m each, or $3.4m x N ( $3+ billion with n=900 ) moderators.

Also in the first line below the header "$3.4m... labour that would cost 2.8 per cent of the firm’s revenue". The study analyzed the work of 900 moderators, so there is likely more moderators on the platform. But as a lower bound, 900 moderators * $3.4 million * 100 / 2.8 would mean $110 billion in revenue. Reddit's last funding round was at a $10 billion valuation, so I leave it to you to decide if $110 billion in revenue makes sense.

There are probably thousands or tens of thousands of moderators on reddit. Obviously it's not 3.4mm per moderator.