Well, what counts as existential? The paper isn't suggesting that climate change will cause humans to go extinct. There's no doubt that it will have very bad consequences if not mitigated, but to a lot of people, being unable to eat meat would itself be a climate disaster. This is kinda what I was getting at - their analysis of what we "have" to do has smuggled in a lot of assumptions about human desires which aren't universally shared.
I suppose I don't disagree, but I don't think realistic vs. unrealistic is the right way to frame this. Questions like "should we eat meat" or "should we promote having kids" are huge, cross-cutting issues which strike to the heart of what we value as a society. The idea that you could analyze them solely in terms of the climate, that climate concerns would compel us to ban meat if it were politically feasible, is already fundamentally confused.