I generally agree with the critique that the broad left, whether that's DNC or the PSL or any random green party, has a blind spot on animal rights. However, I question the authors priors when they write claptrap like the following:

However, utopian socialism is more feasible than Marxism because it imagines a post-capitalist society that does not depend on completely dominating nature, implementing full automation, and somehow instituting a complex, libertarian social order at a global scale.

Regardless of any factionalism from the author, personally I believe the absence of animal rights/veganism from the platform of major or alternative left wing parties, from most climate change analysis, or class analysis stems from carnists and vegetarians not being willing to do the mental work of confronting their own biases. All the ideological mumbo jumbo with $5 words and marxist history and "wut about indigenous people" red herrings are just cover for people's intellectual momentum and cowardice. Engaging with the superficial history of the lack of veganism in the left is interesting but in my opinion is a fool's errand without engaging with the unspoken factors.

In reality, most people are carnists. Meat and animal products are culturally engrained, are calorie dense, many people like the taste, and are often status symbols. There are a lot of incentives to consume animal products. Veganism is not culturally engrained and is usually a negative status symbol.

It is also extremely possible for an individual to make changes to their own life with respect to veganism and in fact the ideology demands it. In contrast, in anti-imperial activism protestors may rail against empire but have minimal if any actual control over where bombs are dropped, and the material benefit they derive from being part of the empire is indirect and probably small. It's easy to show up to an antiwar protest because you probably aren't the one dropping bombs on kids, and the benefit you derive from someone else doing it is probably small. On the other hand, carnists directly and individually benefit from carnism through the positives I mentioned earlier and do so multiple times every day. Being an antiwar activist might only take up a couple hours a month, while being vegan takes time every single day.

Confronting this contradiction means making real changes to one's own life and taking real social costs (albeit many benefits as well). Most people don't want to do this, and a small fraction can't or are limited for some reason or another. Everything else is motivated reasoning.