This study misses two major things that need to be considered to properly evaluate a majority vegan diet for the population vs a diet that has meat. Whatever your personal thoughts, we should be able to agree that we don't have a full picture without this information.
What are emissions, land use, and water use going to look like if vegan food production is scaled up to provide the same calories that a diet with meat has? This is a nontrivial consideration especially since meat is more calorie dense. You will need a larger quantity of vegan options to match a caloric equivalent of meat.
Humans need amino acids that are only found in meat for our full health. This is easy enough to counteract by taking vitamin supplements, but if the entire world needed to take these supplements regularly, what sort of emissions and water use would the scaled up production have? Is manufacturing a high quantity of necessary vitamins going to be better environmentally? I honestly don't know.
Assuming we want the global population to have at least the same food access and nutrition as today, these are questions we need to know the answer to. Maybe the points I've raised are easily addressed without significant emissions. That would be fantastic -- we just need to have a full picture.
I believe both of these questions have been thoroughly answered by the scientific community.
You're right that meat is more nutrient dense than plants. But if we were to replace meat production with crop production for human consumption at scale, we would be averaging far more (I think on the order of 10x) human calories per acre.
When you replace a beef farm with vegan food production, you're not just planting crops on the beef farm. Each of those cows eats for years--crops that humans generally wouldn't eat grown on other farms specifically as livestock feed. You need much, much more land and resources to produce 100 calories of meat than you do to produce 100 calories of vegetables.
Most vegans don’t need supplements. Just check your b12/iron every 2-3 years
This is false. Your B12 reserves will deplete slowly but surely if you don't supplement. Iron, ferritn, and a complete blood count containing hemoglobin and red blood cell indices should be checked more often than once 2-3 years. Many vegans will need supplements, especially young women.
The first point is a non issue. If we just stop for a second and think about it, the calorie output of the animals has to be less than the calorie input. Otherwise they would generate energy out of thin air. The bigger thing is that they are also homeothermic animals, which means that majority of the input calories are used to regulate body heat and not to generate output calories (aka meat). From calorie persective, going through homeothermic animals is pure waste of energy.
Humans need amino acids that are only found in meat for our full health. This is easy enough to counteract by taking vitamin supplements
There are no aminoacids that you can't find in plants. And there are no aminoacids that can be replaced by vitamins.
Iron deficiency is a real possibility when eating a vegan diet, but B12 deficiency is a certitude for those who don't supplement.
I was fully vegan for about 2 years. Being a clinical biologist, I've been having blood tests done regularly and inspecting my own blood smears. After a year or so I've began to see my hemoglobin and ferritin levels drop, so I started trying various iron supplements. Only one of them worked (a sucrosomial iron supplement) and started raising my hemoglobin. Sadly, I deemed it to be too slow and too expensive, so I started eating eggs again, occasionally some fish and chicken. After a few weeks I started gaining muscle mass and I saw some drastic improvent in my fitness levels (I guess I was also underestimating my necessary protein intake). After a year of eating meat and eggs, my ferritin has normalised, and so has my hemoglobin and erythrocite indices.
After my experiences I'm still not going to discourage people from at least trying a vegan diet. I've seen enough vegans who are healthy, and I've seen people for whom a vegan diet is insuitable. It's ok to quit if it affects your health.
You seem to omit that animal meat doesn't come from nowhere, they have to be fed quite a lot to produce a bit of meat (the ratio is 1/8 if I'm not mistaken, I'm on my phone and can't check sources rn). Therefore fields dedicated to animal agriculture could be instead used for human nutrition. That's actually the core of the environmental problems with animal agriculture
Animal in farms are already being fed suplements in b12. Same as before, production for those could be redirected to humans instead