The 'Centrist is a slur!' thing is hilarious, but as with 'TERF is a slur!' it is both true and THE ACTUAL POINT. Like many on the left, I use 'centrist' to mean 'despicable neoliberal sell-out.' If we used 'moderate' in that way instead, it would quickly come to mean the same.

The reason 'centrist' is a slur is not because there is something inherently offensive about the word. It's because there's something inherently offensive about the people it describes. See also TERFs.

Centrist is a slur and oh, by the way, anyone to the left of me is a bastard.

The second paragraph there has the exact energy of Reginald Perrin's neighbour in the greatest scene ever written in the history of British sitcoms.