I followed the links in the article you posted and got this: https://www.nature.com/articles/s41598-018-22396-5
Let me clarify something: Mnemonists do use working memory, because everyone uses working memory all the time.
If someone asked you “Where is germany?” your brain would keep “germany” in your working memory, while simultaneously searching your long-term memory for your geography knowledge and comparing both until it finds the answer, ie, “Germany is in europe”. When you type in your computer you keep what you want to say in your working memory while you hands type the letters out. Etc.
If the brain was a computer, working memory would be the RAM , which is used to process everything the computer does. The hard drive would be long-term memory. And mnemonics would be using the most efficient way to store and retreive data from the hard drive.
And to answer your original question, I think we could adapt the conclusion of the study to mnemonics (even though the study was about somethings different) like this: The strategies that mnemonists use, and I guess you already know them, are things like
- Association (for instance, associating a person´s name with a trait in their face)
- Use of images (for instance, converting numbers into persons/actions/objects)
- Location (for instance, memory palace)
It would take 30 minutes to explain the basics of mnemonics to someone new to the topic, but it would take days, months, or even years for someone to develop those strategies on their own.