I always thought it wasn't so much that they assumed that you were a retard, but rather that if they frame it as though you are retarded, then other leftists can jump in and make fun of you for being retarded, and since none of them ever engages with good faith, all those jumping in without knowing the facts, can all feel good about dunking on the ignorant, retarded, Nazi, and chalk it up as a victory for progressiveism, regardless of what the original dispute was about.