The answer is no. An llm agent can write a post as well as edit it, and it can be done without the Lemmy instance knowledge or capability to detect.

An llm agent uses a browser to act, so it’s much harder than a normal bot to detect. It can also use normal user credentials if those were provided to it, so I can edit my comment: create by human and edit by ai. Also, of course, true the other way around.