How is it difficult with detectable watermarks? If it has the watermark it is from that specific LLM, if there is no watermark it isn't from that LLM. Unless somebody tampered with the watermark, but that's exactly where undetectable watermarks have an advantage. If you don't notice that it's there you won't tamper with it.
Scott Aaronson worked on that at OpenAI, but GPT-4 didn't use such technology, nor have I seen any other major language model which had the ability of accurately distinguishing model output and human text.
