It's about informing the only person who can get extra information, decide if it's severe and do something about it - the editor.
The author can't (and shouldn't) do anything directly about the anonymous reviewer, all the responsibility, authority and duty is up to the editor, who at least knows who that person is.
Of course. I would do pretty much the same thing (finishing the review process first before complaining), but I would mention the suspicion of AI usage. If that aspect is missed, then it's very much possible the investigation won't lead to the necessary improvement in the organization. It's worth speaking out about such a problem if you are comfortable enough in your career to risk it, because it represents something that's going to rot the entire journal and the scientific community as a whole.
The author can't (and shouldn't) do anything directly about the anonymous reviewer, all the responsibility, authority and duty is up to the editor, who at least knows who that person is.