A lot of community types just simply don't work without a minimum critical mass of members.
Imagine asking a programming question on a software development community of just 5 people. You end up with 3 people who aren't active enough to see the question, 1 person sees but doesn't have an answer and doesn't respond (classic lurker), and one person sees it and responds that they don't know the answer. Now imagine a community of 5 thousand people...it's suddenly much more feasible to even bother asking the question.
Sure, fediverse could exist with just 5 people, but it would be worthless and pointless.
This crucially important caveat they snuck in there:
"Prof Scarborough said: “Cherry-picking data on high-impact, plant-based food or low-impact meat can obscure the clear relationship between animal-based foods and the environment."
...which is an interesting way of saying that lines get blurry depending on the type of meat diet people had and/or the quantity vs the type of plant-based diet people had.
Takeaway from the article shouldn't be meat=bad and vegan=good - the takeaway should be that meat can be an environmentally responsible part of a reasonable diet if done right and that it's also possible for vegan diets to be more environmentally irresponsible.