Yes I have seen AI detectors in the other forum accused someone I know of using AI in which the person is not using AI and for that the AI detectors can't give the 100% accuracy and lastly I didn't see any different between AI and plagiarism. So is there any contradiction here. There was no contradiction but misunderstanding which I also discovered. If someone copied what has been written by another person and bring to the forum and used it as his own there automatically it is a plagiarism be a AI. Unless the person. Edit it to his own way of understanding.
Usually writing that uses AI can be felt using our own feelings when reading it... the general characteristic of writing made by humans is that the sentence structure is sometimes messy... while AI users will get almost perfect writing...
I agree more with the idea that the human brain produces is an original work... whether it will be fixed using grammarly or the word structure will be corrected to make it easier to understand using other tools, what is certain is that it is the brainchild of humans and remains on topic, I don't think it will be considered an AI post, even though AI detects it as non-human-made content...