Because evolutionary fitness doesn't care about what we care about, and even if it did it operates so slowly it hasn't yet finished adapting to us having invented cooking.
That said, I share your skepticism. This kind of story feels almost exactly like the old Victorian (literal) snake oil advertising: https://commons.m.wikimedia.org/wiki/File:Clark_Stanley%27s_...
You could say the same for myopia. But we still make glasses for people, which breaks the natural selection process that would drive that evolution.
And that's being generous, because the food industrialization (removal of fiber, concentration of sugars), and cultural norm of consistent fast food is much more in line with the tail end of millennials than any generation before.
Of course we're not seeing evolutionary pressures for diabetes, when, at the same time we've generally increased life expectancy past reproductive years.
Most basic factoid: Insulin was discovered in 1921 before Penicillin in 1928, before the Polio vaccine in 1950.