I can't find it in RationalWiki.
It's basically an overstatement.
Imagine causal factors a, b and c that cause effect x.
We don't know what their causal weights are.
But someone insists a is the greatest.
I notice a lot of political discussions degenerate towards this conversation-ender.
They're insisting that a is the greatest factor, so that they can drive to the conclusion that a is what needs to be increased or decreased, in order to decrease some social ill or whatever else. That's the motivation to commit the error. It's either empirically true or it isn't, but we don't have that information and they're insisting they know without any evidence.