No doubt cooking makes some plant materials more easily metabolized into glucose.
None-the-less the plant material most widely and readily available during the Pleistocene was predominantly cellulose, which no amount of cooking/boiling is going to make digestible.
From this source:
Humans are known for sporting big brains. On average, the size of primates’ brains is nearly double what is expected for mammals of the same body size. Across nearly seven million years, the human brain has tripled in size, with most of this growth occurring in the past two million years.
…With some evolutionary irony, the past 10,000 years of human existence actually shrank our brains. Limited nutrition in agricultural populations may have been an important driver of this trend. Industrial societies in the past 100 years, however, have seen brain size rebound, as childhood nutrition increased and disease declined.
All those selectively bred and cultivated carbs, cooked or otherwise, didn’t do much for our brains. And we know they did a lot to our metabolic health. Again, where is the ‘good carb’?