Yes, we have a cecum
by name, as do other hypercarnivores including cats, but its design in no way suggests that humans developed to primarily eat plants or even digest them in an efficient way. Even the article you cited states explicitly that the function of the human cecum is different than that of herbavores, and it's dubious whether it should even be referred to as a cecum.
If you look at the digestive tract of herbavores, such as cows, deer, horses, and rabbits, their cecum is a distinctly large and is typically considered a separate digestive organ.
Besides absorbing salts, the human "cecum" is a glorified dead end to the large intestine. A human could easily live without one if it were removed. It would have nearly no impact on our ability to swallow cellulose and excrete it undigested. Removing the cecum from an herbavore like a horse would be an entirely different story because they need it to process cellulose.
> And we can digest plants just fine without having to ferment them.
This is not correct. Humans are entirely ill-adapted to digesting plant material. You will find no credible source that suggests that our digestive tracts are good at digesting cellulose, which makes up the bulk of plant material because it composes their cell walls. Just because we can swallow plants and poop them out without necessarily getting sick or dying doesn't mean it's at all optimal.
At best, we can ferment a small amount of cellulose given that it has been sufficiently broken down through cooking. Even if we were better at fermenting it, we are extremely poor at grinding it and are bad at harboring microbiota that produce cellulase enzyme.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8661373/
Can we derive energy from plants? Of course. If those plants are abundant in sugars and proteins, then we can access those. That is possible in present day because agriculture and selective breeding made edible and energy dense plants available and ubiquitous. Such plants did not exist the way they do today, were usually scarce, only available seasonally, and were energy-expensive to obtain and prepare.
> There's clearly a kind of madness in all those diets that starts to become dangerous when it becomes a fad.
Depends on if those diets are actually dangerous. What's more dangerous than believing that humans are developed to primarily eat meat is to ignore all available scientific evidence and promote fanciful ideas around plants being good or even better for humans. I await the day that a child dies from being fed plenty of meat.
Meanwhile...
https://www.independent.co.uk/news/world/americas/vegan-coup...
https://www.vice.com/en/article/wjqbem/judge-convicts-parent...
It's as if the human body is optimized to process animal material and is not so good at processing plants.