What's the evidence for this? I think even if it is true, it is probably misleading, in that most leftists also just reject the claims mainstream economists make about when taxing the rich will reduce aggregate welfare

To support this claim, we can examine the work of analytical anticapitalists such as G. A. Cohen and John E. Roemer. Both of these thinkers have developed their critiques of capitalism from a foundation of egalitarianism rather than from a perspective primarily concerned with maximizing overall social welfare. Their theories focus on issues of fairness, justice, and equality rather than on the utilitarian consequences of different economic systems.

Similarly, widely cited figures such as Thomas Piketty and John Rawls have provided extensive critiques of capitalist systems, and their arguments are largely framed in terms of egalitarian concerns. Both have explicitly advocated for significant wealth redistribution, even when doing so might lead to efficiency losses or other negative utilitarian tradeoffs. Their work illustrates a broader trend in which anticapitalist arguments are often motivated more by ethical commitments to equality than by a strict adherence to utilitarian cost-benefit analysis.

Outside of academic discourse, the distinction becomes less clear. This is because most people do not explicitly frame their economic beliefs within formal theoretical frameworks, making it harder to categorize their positions precisely. I also acknowledge your point that many socialists would likely disagree with my characterization by denying the empirical premise that wealth redistribution can reduce aggregate utilitarian welfare. But this isn't very compelling evidence in my view, as it is common for people among all ideologies to simply deny the tradeoffs inherent in their policy proposals.

What I find most compelling here is that, based on my experience, the vast majority of anticapitalists do not ground their advocacy in a framework that prioritizes maximizing utilitarian welfare. While they may often reference utilitarian concerns in passing, it is uncommon for them to fully engage with mainstream economic analyses of the costs of taxation and redistribution. When anticapitalists do acknowledge these economic arguments, they tend to dismiss or downplay them rather than engaging in a substantive, evidence-based debate within that framework. Those who do accept the mainstream economic framework and attempt to argue within it are generally better categorized as liberals or social democrats rather than strong anticapitalists.

Of course, the distinction between a liberal who strongly supports income redistribution and an anticapitalist is not always sharply defined. There is no rigid, universally agreed-upon boundary between these positions, and I acknowledge that some individuals who identify as anticapitalists may not fit neatly into the categorization I have outlined. However, my original point was intended as a general observation rather than as an exhaustive classification of every nuance within these ideological debates.