Writing is an exercise that helps thinking and reasoning about a subject. Delegating it to an LLM is the same as delegating it to another person who writes faster and knows the subject well. The end result is the same, the weights have been moved up 3 series of 15 times. But you didn't gain any muscle, because you didn't lift the weights yourself. You won't learn or even think about a topic if you delegate writing about it to a machine.
--
On a separate note, I also think it's naive to think that the LLM is reasoning about things the same way you would and writing the same things with the same conclusions. If you _read_ the LLM's work, you might get that impression. But your own writing could have spawned different questions along the way, leading you to read on different topics or connect different ideas. Just try asking two people about some complex topic and see if they come up with the same writing or not.