I'm looking at this from leftist principles, not vegan principles, due to the post title conflating the two. To answer your question, I personally only permit spiders and humans in my house and the practical ability to evacuate would affect what I could save other than myself from a fire. I understand the energy efficiency from consuming lower in the food chain, I'm not disputing that. I am offering a genuine alternative in lab grown meat that would avoid both. I'm saying that we as a society could do better, I'm not strawmanning saying you must be perfect to be a true vegan, or that makes your choices not good enough. Each person reducing meat consumption does add up to real benefits, and I'd agree we should abolish cattle industry subsidies, ag-gag laws, and restrict factory farming, common goals for leftists and vegans. But all that doesn't mean that it's necessary, as OP's title suggests, to be vegan to be leftist or vice versa.

I'm bringing a thought that sounds absurd to you in response to an absurd idea to me that veganism = leftism. It has commonalities but one does not require or imply the other.

I think moral comparisons are arbitrary and differ from person to person, I think there are positive and negative directions that can be aligned or askew between people. I get all the benefits from environment health and societal welfare by choosing plant over animal products, but each person should draw the line where they want initially, then continue to improve where they feel like they can after constructive discussion. Shaming or strawmanning people's choices is not my intent, I'm suggesting how as a society we can further improve and reduce impact from agriculture while still being able to sustain a healthy life for ourselves. The one, single, solitary thing I push back on is the gatekeeping aspect of it from OP.

To give you an example: If you cut off my arm, and then manage to keep that arm growing skin cells forever to harvest, it was originally part of me but not really a being anymore and reduces the need to cut off anyone else's arm. It's analogous to that.