> it likely already is as the true cost of rainforest loss is externalised - rainforest loss to soya fields that is 70% fed to cattle amounts to the same thing: rainforest loss for meat

Last time I've read related news those lab-grown muscle cells were supplied with calf blood serum. That makes your sustainability speculation sound more like wishful thinking.

I'm sure they're working on that issue, but we're not even close to being able to feed billions with vat-grown meat. And you need to feed that meat with some nutrients too, probably plant-derived ones. So on a per-calorie basis feeding the humans with those plant-derived nutrients directly would probably be more efficient, if your goal is to reduce carbon footprint.