It is a perfectly fine rhetorical device, and I don't consider a text that just has that to be automatically LLM-made. However, it is also a powerful rhetorical device, and I find that the average human writer right now is better at using these than whatever LLM most people use to generate essays. It's supposed to signify a contrast, a mood shift, something impactful, but LLMs tend to spam these all over the place, as if trying to maximize the number of times the readers gasp. It's too intense in its writing, and that's what stands out the most.