You dropped the context of the discussion. We aren’t talking about machines, but humans. That is a different usage of the word “difficulty”, meaning something

You dropped the context of the discussion. We aren’t talking about machines, but humans. That is a different usage of the word “difficulty”, meaning something requiring more work to achieve an outcome, but that is not the relevant meaning in this context. In the context of this discussion, “difficulty” is being used to describe the level of significant and sustained focused mental effort to grasp and integrate a new concept into one’s existing body of knowledge.