In general, any product or service that makes exaggerated or false claims of effectiveness without providing concrete evidence to support those claims could be considered a form of "snake oil."
Just because we don't know all the workings of something doesn't mean we can't effectively use it, or measure how well that approach of use is valuable or repeatable for similar uses.
For example, while we have a good understanding of the basic principles of magnetism, there is still much that we do not fully understand about how magnets work, and ongoing research is focused on unraveling these mysteries and expanding our understanding of this fascinating natural phenomenon.
That's not to say we don't know how to use magnets to make sound, however.
Prompt engineering, in my opinion, is a process of optimized querying of a frozen model. The approach can be augmented by using "hot" data from vector databases or other types of text storage engines. The approach to picking the right content for prompt building is not "snake oil", but based on well known processes including autocomplete, synonyms and related terms, personalization, and query expansion.