Why doesn't he feel equipped? Is it an age/speed-of-learning thing, or more of a, there's too many new concepts and not enough time? Genuinely asking, not tryi

Why doesn't he feel equipped? Is it an age/speed-of-learning thing, or more of a, there's too many new concepts and not enough time? Genuinely asking, not trying to be snarky. My prior assumption is typically that a capable engineer in one language or stack can learn another relatively easily.