Well there is a lot of science that supports ketogenic diet, and the benefits of animal fats and proteins. There’s also the information about the nutrient bioavailability and density in meats vs the lack thereof in many plants due to anti-nutrients. We also have a good idea of what sugar and insulin spikes do to people over time. And there is also a lot of studies on intermittent fasting as a beneficial eating modality. So people just put 2 and 2 together and basically you have the carnivore diet. There are also studies on cultures that have eaten primarily meat and fat as well.
I think Dr Gregers is just promoting an ideology, but the science does not support it. And since Veganism has been around for a while, we also have their experiences as a data point, and there are a myriad of health problems associated with their way of eating. Just the fact that their diet requires supplements to make up the nutritional deficiencies from lack of animal products is enough reason to take pause.
So I would say it’s the other way around - those who switch to plant based often see improvements because of less junk and processed foods, not because of the diet. Also, seeds and nuts actually require a lot of processing just to be edible and not acutely toxic, like soaking and other methods. On top of that, something like oxalates are not reduced by most of those methods anyway.
And of course you have the basic logic that if you take away farms and such, and place yourself in a forest, good luck finding enough non-animal food naturally growing around you to sustain yourself. Almost everything that moves can be eaten, but vast majority of plants cannot. So it just makes sense that outside of very special and seasonal circumstances in specific parts of the world, it was just much easier for humans to find and eat animals for most of our past.
biscuits, sweets and snack food. It has been meat e.g. mince, pork chops, bacon or salmon, pan-fried [butter or lard] instead. I have also eaten greek yoghurt and cheddar cheese when I feel the need for a snack and don't want meat sitting in my stomach late in the evening. Drinks have been lots of tap water, tea, decaf/coffee and hot chocolate [naughty because of the sugar in it but 
]. My diet has been awful for 20 years and I have practically lived on those drinks. It is going to be a tremendous wrench to give those up so that day can wait until I am strong enough to handle it. I am eating more regularly now and although simply, a better quality of food without all that processed junk so I shall celebrate my improvements and not try to scale the mountain in two seconds. 
Perhaps you could just try it for a period? I more or less tried what I am doing on a whim. My nutrition was so poor anyway I thought that it probably couldn't hurt. Try it for 3 months and see how you get on?
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