LLMs are not defacto bad at writing and are not defacto distinguishable from human writers. Lots of people are just lazy, never customize anything, and tell chatgpt to make them a post about x to be a "thought leader". Most are even lazier and blatantly rip off some else for their own means.

However, it is perfectly possible to have an LLM imitate an existing corpus of writing (yours or someone else) and with a good prompt, idea, and editing, to produce high quality writing (in every sense of the word) with an LLM.