The examples in this book are extraordinarily simple, and covers material that many proof assistants were designed to be extremely good at expressing. I wouldn't be surprised if a LLM could automate the exercises in this book completely.

Writing nontrivial proofs in a theorem prover is a different beast. In my experience (as someone who writes mechanized mathematical proofs for a living) you need to not only know the proof very well beforehand, but you also need to know the design considerations for all of the steps you are going to use beforehand, and you also need to think about all of the ways your proof is going to be used beforehand. Getting these wrong frequently means redoing a ton of work, because design errors in proof systems are subtle and can remain latent for a long time.