Like where would the need for the fiber come from evolutionary.
All those were very high in fiber. I believe it's estimated paleolithic humans are over 100 grams of fiber a day, whereas I believe the recommended intake today is 35 grams, which less than 2% of Americans meet.
So yes, the Paleo diet is largely bullshit. No, humans did not eat fatty farmed meats. They barely ate meat at all.
And it's also well-documented that the average Western diet is highly deficient in fiber and that this is a thing which has gotten much worse in the last 75 years.
There also seems to be at least some light evidence that there may be generational effects - that the starting point of your gut is already bad if your mother's was.