Astroturfing implies that a corporation or government agency with large amounts of funding are paying individuals or bots to spread misinformation for their employer's financial or strategic benefit.
You might not know this, but there isn't a "Big Vegan" industry with deep pockets to financially support astroturfing. Agrobusinesses that grow vegetation make more money off the meat industry than they would if they centered their produce around vegetarian or vegan diets. Businesses that do cater to vegans barely manage to scrape by and have no margins to support social media manipulation; they barely even have budget for conventional marketing.
What you're actually witnessing is legitimate grassroots efforts to inform people about the harm that the meat industry causes. You see "astroturfing" doesn't mean "a lot of people are saying things I don't like". It actually means "grassroots campaign but fake", hence the name "astroturf", which is a fake kind of grass.