There's very little evidence that completely abstaining from meat (animal based protein) brings any meaningful advantages from the perspective of life span. In fact in general, it's far more difficult to build a complete diet eating vegan only. I of course do think there's a moral argument to a vegan lifestyle, and am a definitely a strong proponent on that end.
Personally, I think a vegetarian diet strikes a better balance that's also more sustainable in the long term, assuming that you also do care about maximizing life span as well.
Your link has some great information. Plants are highly nutritious, and people who don't get enough of them are probably unlikely to be as healthy as they could be.
That, however, doesn't preclude meat being beneficial as well. It's not an analog one is better than the other, and it doesn't mean you can't build a complete diet being vegan either. It's possible, in fact, to build a complete diet eating meat only as well if you really wanted to. Generally though, it's far more difficult, and you have to be pretty meticulous about what you eat in addition to probably having to supplement additional micronutrients into your diet to do so.
B12 you can get from a pill. The same stuff they inject into beef. [1]
So if you skip eating red meat, you can avoid the toxins from cooking, such as HCA and PAH [2], the exogenous cholesterol, atherosclerotic carnitine [3], hormones, etc. I think that's a better way to get B12.
If you feed cows corn all day every day, yes they need B12 injections. I buy beef from a local butcher and the beef comes from our local farm of pasture-raised cows. It's more expensive but because of this it limits my beef intake to a few times a month.
liver == steak * 1000
Only problem is being able to stomach liver a few days every month. I know it's a super food but I'll be damned if I can eat it more than once every few months.
Personally, I think a vegetarian diet strikes a better balance that's also more sustainable in the long term, assuming that you also do care about maximizing life span as well.