Basically liberalism tends to be capitalist, whereas leftism tends to be anticapitalist. Historically, there’s been a pattern of centrists and capitalist allying with authoritarians and fascists against leftists, so the insult boils down to “you’d support a fascist before you went against capitalism”
Disagree: if you start at the top and work down, then the wet spots don’t drip down onto the parts you’ve already dried.