I don't see how they conclude so. Major meat (as in steaks, burgers, etc.) in agricultural societies before the industrial revolution would be a luxury saved for special occasions.
So I always maintained, but I made no headway at all.
Plus, while puls and bread were indeed the mainstay of the diet of the Roman people, a paper a couple of years ago examined the waste in Pompeii, and found that even the poor ate a pretty varied diet, with lots of fish, lentils, olives, fruits, etc. as well as cereals and vegetables and some salted meat. I remarked at the time that it seemed like a pretty good diet to me, but was told it was a one off because it was a rich coastal city. People believe what they want to believe, even when confronted by contrary evidence.
https://www.livescience.com/42309-food-eaten-by-pompeii-residents.html
In the intervening time we've had more papers from various regions and they all show the same thing. In particular, it now seems clear that while beef wasn't on the menu in the urban areas, pigs were apparently commonly kept, as I also always maintained. They can be kept in a pen,fed from kitchen waste, and the meat can be easily salted for preservation. During the occupation in Italy during the second world war pigs kept in little hidden pens in the woods, along with chestnuts, kept people alive.
"
Seneca, for example, complains about the hawkers who frequent the bathhouse below his apartment in first-century Rome, describing the noise of the “pastry-cooks with their varied cries, the sausage dealer and the confectioner and all the vendors of food from the cookshops selling their wares”.Other writers liken the poor poetry or speeches of their competitors or enemies to the shouts of retailers, the proverbial ‘fishwives’ of their day. Martial likens the wit of a friend to that of a “vendor of boiled chickpeas”, or the slaves of the fishmongers, or the “bawling cook who hawks smoking sausages round stuffy bistros”."
That's not to say they were great meal eaters, because they weren't. The diet then is much like the Mediterranean diet today in its proportions.
As for the Roman army, they were fed very well, and not only on puls. Wherever the army was settled they built fish ponds, kept their own pigs, cattle, ducks etc., and also imported olive oil, wine, etc. Even oysters, a great favorite, were brought in, kept in salt water tanks.
https://romanrecruit.weebly.com/food.html
"In particular, the exploitation of pigs and chickens seemsto be strongly influenced by the appearance of the Roman troops. This may be an indicationthat the Roman army used these animals as a reliable, easily transportable, rather undemandingand fast reproductive protein resource, in times of campaigns or in unfavourable circumstancessuch as lack of local resources."
http://uprav.ff.cuni.cz/?q=system/files/feeding roman army_05.pdf
Of course, when they were marching it was a different story.
Despite the numerous papers written on the subject in the last few years, if you google the subject you will still find article after article saying they lived on gruel.
I don't know why anyone would have thought the legions could have conquered most of the known world with scrawny, anemic, puny men. Organization and strategy will take you only so far.