The big difference is feedback loop. With prompt engineering you can change your prompt slightly and immediately see if benchmarks improve. With SEO you had to wait at least days, and even then it was impossible to rule out external effects like others adding/removing links to you.
Anyone who had to deal with retraining models understands the pain of retraining. What prompts offer as a feature while at the surface may seem superficial, they are groundbreaking, as they completely remove re-training and the entire ML-Ops deployment lifecycle.
