Not a 'fraud', in any way. Just a bad decision, and possibly poorly thought out in addition.

I know nothing about the technical side, though the fact that Rimu pivoted to a different explanation when pressed has me skeptical. But it very well could be that voting uses up some significant amount of 'the commons' in federating votes. I suspect the actual impact is minimal, though, considering that Piefed.social is the only instance that has even broached the issue, so far as I know, in the three years I've been on the Fediverse. And, as mentioned before, it's trivial to 'get around', resulting in the same vote-federation burden as large numbers of votes with one account.

The other explanation offered by Rimu, which I regard as much more likely, is Rimu's claim that it's in some way unfair for a small number of users to make up such a large proportion of the Fediverse's activity. I take him at his word on this - I think he genuinely believes that. No fraud involved. Just a view that power users are some form of 'influencer' whose activity is undesirable because it 'sways' the community towards the opinions of the most active users.

Rimu, I suspect, thinks that cutting down the tall poppies will just create a more even distribution. Instead, I suspect, it will either result in the former distribution proportion reasserting itself, just with a lower cap and a lower average (thus requiring another reduction of the cap, to ensure a small number of users don't 'dominate' the activity - repeat ad infinitum, or until the instance is dead); or the 'more even' distribution being significantly lower in general from decreased activity, as the core function of social media - interaction with others - generates more activity from others as a knock-on effect. It's self-feeding.

Or self-starving, in this case.