I didn't manage to read all the way through the entire study (a lot of the technical jargon goes over my head), but a couple of things stand out to me.
First of all, they're lumping vegetarianism and veganism together. Vegetarians (by today's definition) consume animal matter, just limiting it to dairy and eggs, which provide certain nutrients in forms which can be absorbed. Vegans don't eat animal matter of any kind, meaning their diet lacks certain micro-nutrients not found in plant matter in a form which can be absorbed.
It would be very possible for a vegetarian to consume sufficient proteins to attain the same level of animal protein consumption as a meat eater, and that's a limitation of this study. In my opinion, it would have given far more accurate results if the comparison had been between animal protein consumption and plant protein consumption.
Perhaps another study for another day though.
One thing that really stood out to me (which is something I've often said) was this though:
Before agriculture was introduced (circa 11–9000 years ago), human ancestors could not grow, harvest and store the majority of plant-based products as the staple food. Plant foods are mostly accessible only in particular seasons of the year.26 Contrariwise, animals, including large game, small animal, fish and some insects, could constantly provide humans with meat as the staple food.
In other words, it wasn't possible for plant based foods to be THE primary source of food during a good bit of the year before the era of agriculture began. Even then, preservation methods for harvested foods were iffy at best. If every single piece of fruit was not dried sufficiently, or had picked up mold spores from the air, they could result in ruining an entire storage container of dried fruit. Up until the last couple hundred years when canning methods were first developed, it was incredibly difficult and time consuming to preserve sufficient agricultural foods to prevent total starvation during the rest of the year. Logic says that humans needed
to eat animal products to survive the long winters, even after agriculture was developed.
For the most part that does mean eating meats of some kind, since most egg laying occurs in the spring and summer, and most agricultural animals providing milk would have weaned their young before the winter. So even strict vegetarianism (eggs and dairy) is a completely modern construct.
Not to mention that if they were eating eggs and dairy, what exactly were they doing with calves and lambs that were weaned and the chicks/ducklings/goslings that hatched from any uneaten eggs? What would have been the point in keeping all those, rather than eating them?
Simply put, humans have always eaten animal products - mostly meat - from the beginning, and no idealized society that eschews all animal products would have survived even the first winter.
Hence as WereBear pointed out:
If they get all the protein they need from plants, why would they do that? Not a single vegan society in all of history.