Reddit is different for multiple reasons.
It has karma, the fediverse doesn't.
It has millions of daily users, the fediverse doesn't.
It's a publicly traded company with a profit motive, the fediverse isn't.
It allows paid advertisements, the fediverse doesn't.
There's therefore a monetary incentive to farming karma on reddit, there isn't on the fediverse.
Some people farm karma on reddit accounts just to sell them to advertisers or propagandists. This wouldn't work on the fediverse for the above reasons, and because there's no market for it for the same reasons.
Reddit feeds are governed by closed-source algorithms designed by a centralized company, fediverse feeds are not.
And lastly, you said yourself that the guy used secondary accounts to boost himself. This is voting abuse and should be dealt with whenever it can be positively identified.