>high protein intake from beans and tofu and a lot of vegetables+seeds

Saying "high" protein implies that the protein:carb and/or protein:fat ratio is high. Beans have a much higher carb to protein ratio and tofu/seeds have high fat to protein ratios. It is close to impossible to eat a high protein diet that is vegan. The only way to do so requires large quantities of highly processed foods (saitan, etc...)

"Processed" isn't a well-defined term to begin with, but it's strange that you seem to implying that seitan, but not tofu, is highly processed. The steps from soybeans to tofu are: 1) dry soybeans, 2) soak soybeans, 3) puree soybeans, 4) boil soybeans, 5) separate pulp (okara), 6) coagulate with a coagulant compound, 7) press into block. The steps from wheat to seitan are: 1) mill wheat into flour, 2) wash flour to remove starch, 3) knead glutenous dough, 4) boil. That's fewer steps of "processing" for the latter. You can quibble a bit (in particular, if one buys vital wheat gluten powder, rather than making seitan directly from flour, add a drying step and a rehydrating step after step 3), but at the least, they're in the same league. Moreover, tofu production requires a somewhat exotic ingredient (the coagulant), whereas anyone can make seitan at home from a pantry staple.

Also, it's a logical leap from "requires large quantities of highly processed food" to "close to impossible". Whether seitan is "highly processed" or not, it's tasty, and eating a lot of it isn't close to impossible.

Humans cannot live on really "high protein" food, which would eventually cause death. The majority of the energy intake has to come either from fat or from carbohydrates or from both.

So, when speaking about humans, it makes sense for "high protein diet", to mean a diet where proteins make a much higher percentage of the food in comparison to lower protein diets, like those recommended by USDA.

Therefore, when speaking, for example, about a diet where all the cereals (which have very low protein content) are replaced with legumes such as lentils, dried peas or beans (which have a protein content of up to a half of the carbohydrate content), it can be said that the legume-based diet is a high protein diet, as that would be true relatively, in comparison with the food eaten by most people, where the cheaper cereals are the main component.

Why does “high protein” food cause death?

Because the metabolism of the proteins requires not only oxidation reactions, which are shared with the oxidation of carbohydrates and fatty acids, but also many other chemical reactions, mainly involved in the transfer of the amino- group between various molecules, until eventually it accumulates in urea, which can be excreted.

The total capacity per day of performing those reactions is not sufficient to allow the production of enough energy from proteins to sustain a human, unlike in the carnivore animals.

When too much proteins are eaten, some intermediate molecules accumulate in excess, instead of being converted into excretion products.

Before the 20th century, when this was not understood yet, there have been many cases of illness or even death among people who ate too much lean meat, mainly among European hunters or explorers of the Northern regions, until they learned to copy the methods of the natives, e.g. by eating pemmican, in which the lean meat is mixed with enough fat to provide sufficient energy without having to eat too much proteins.

Like for any food component, for proteins it is bad both to eat too little and to eat too much, but there is a large enough interval of protein quantities per day that are all right.