The companies that run LLMs can also use this for PR to calm concerns from the public about the proliferation of such content.
From a CS perspective I can't think of any way to have a watermark that couldn't be easily defeated through additional processing. LLM dependent people currently take pains to remove em dashes so this new wrinkle has some of them in a panic.
But, why not just be honest and say you used and LLM? Why so bashful?