At this point, we're very much at an exploratory stage of LLM queries - you could of course be a ML/DL researcher or engineer that's intimately knowledgeable with the current architectures, but still - they're very large and very complex due to the sheer parameter size, so you'd still have to map out what inputs will predictably give what outputs on a finished model.

I'd imagine that being a "prompt engineer" entails finding out and mapping the structures that gives you the desired result. Think of it as a novice user of search engines VS expert user of search engines.